How Do You Calculate Build vs Buy for AI Tools?

What is the break-even formula for building an AI tool?

The break-even point is the month when cumulative buying cost becomes higher than cumulative building cost. Calculate it with: break-even months = (build cost - buy setup cost) ÷ (monthly SaaS cost - monthly internal operating cost). If the denominator is zero or negative, building does not recover its cost through subscription savings. The inputs deserve more scrutiny than the arithmetic.

Start with a 24-month comparison, unless your buying contract or product roadmap gives you a better horizon. Put every cost in the same currency and include the time of the people who will keep the tool alive. A cheap prototype can still become an expensive internal product.

Which costs belong in the build calculation?

Build cost is the one-time work required to reach a version people can use safely. I would put these lines in a spreadsheet before asking an engineer for an opinion:

• Discovery: interviews, process mapping, acceptance criteria, and a decision about what the model is allowed to do.
• Delivery: application code, prompts, retrieval, integrations, authentication, permissions, testing, and deployment.
• Data work: cleaning source documents, access rules, evaluation examples, and red-team tests for bad answers.
• Launch work: training, migration, documentation, and a rollback path.
• Legal and compliance work: data-processing review, contract review, and approval for sensitive use cases.

Then add monthly internal operating cost. That includes hosting, model or API usage, logs, monitoring, security review, backups, incident response, maintenance time, and any model retraining or evaluation refreshes. AWS describes cost optimization as an architectural concern that should be tied to business objectives, not treated as a late billing exercise. That is the right instinct for an AI tool too: measure the system after launch, when the invoices and support requests arrive.

Use your loaded internal cost, not a fantasy contractor rate. If an employee spends 80 hours on a build, the spreadsheet should show 80 hours multiplied by the cost your company uses for that employee's time, including salary, benefits, and overhead where finance includes them. If you do not know that number, use the finance team's standard rate and label it as an assumption. Do not hide the uncertainty inside a decimal.

Which costs belong in the buy calculation?

The buy side starts with the subscription, but it does not end there. Count the seats or usage tier, implementation, data migration, configuration, integration work, admin time, user training, support, and the cost of changing vendors later. SaaS pricing pages often make the first line easy to find and the last five lines your problem.

For a current reference point, OpenAI lists ChatGPT Business standard seats at $20 per user per month when billed annually and $25 per user per month when billed monthly on its business pricing page. OpenAI also states that API usage is billed separately from ChatGPT subscriptions. Those are two different buying decisions, so do not compare a per-seat chat product with an internal API application as if they were the same SKU.

A simple buy formula is:

24-month buy cost = setup + (monthly subscription x 24) + internal admin

The 24 in that expression is an example horizon, not a universal rule. Change it when the vendor contract, expected product life, or renewal risk makes another period more honest. I have seen spreadsheets use a 36-month forecast for a workflow nobody had funded beyond the next quarter. That is accounting theatre with nicer formatting.

How do you calculate the actual break-even month?

Make one row per month. Put one-time build costs in month zero, then add the internal monthly operating cost. In a second column, put SaaS setup costs in month zero, then add subscription and administration. The first month where cumulative build cost is lower than cumulative buy cost is the break-even month.

Here is an illustration with deliberately round numbers. Replace them with your own figures:

• Build: $18,000 of internal delivery work, plus $700 per month to operate.
• Buy: $2,000 of setup, plus $1,800 per month for the subscription and administration.

The monthly saving after launch is $1,100. The extra upfront build cost is $16,000. The break-even point is 14.55 months, so the build option has not recovered its extra cost during the first year. This example is arithmetic for the model, not a market estimate.

That result still does not make build the winner. Ask whether the internal tool will remain useful for the full comparison period, whether the team can support it during holidays, and whether the bought product includes a feature you would otherwise need to build. Break-even is a gate for discussion, not a permission slip.

What changes the calculation for an AI tool?

AI adds costs that ordinary SaaS comparisons often miss. Model usage can rise when employees discover a new workflow. Retrieval indexes need refreshes. Prompts and evaluation sets need version control. A model vendor can change a price or retire a model, and a tool that looked stable in January can need testing again in April.

Separate variable AI usage from fixed software cost. OpenAI's business pricing documentation says API calls are charged separately from ChatGPT subscriptions, and its API pricing page publishes token-based rates by model. Put expected input and output volume into a usage row, then add a high-usage case. Do not use the average month if one busy sales campaign can consume the budget.

On the buy side, ask what happens to your data, exports, permissions, audit logs, and integrations if you leave. On the build side, ask who owns the code, dependencies, credentials, evaluations, and on-call decision. The internal product needs an owner with time on the calendar. “The developer will look at it” is not an operating model.

When should a small company buy instead of build?

Buy when the workflow is common, the vendor meets your data requirements, and the subscription remains cheaper than owning a product team for the period you actually expect to use it. Buy when speed matters more than perfect fit. A 40-person company rarely needs to own a generic help-desk layer just because it can connect an API.

The strongest case for buy is a process where vendors already carry the boring burden: permission handling, billing, uptime, backups, user provisioning, and routine feature work. AWS's Well-Architected guidance treats operational excellence, security, reliability, performance efficiency, and cost optimization as separate concerns. Your spreadsheet should do the same.

When should a small company build instead of buy?

Build when the workflow is a genuine differentiator, existing tools force manual work into every case, or the data and control requirements rule out the available SaaS products. Build when the same internal capability will serve several teams and the reuse is real, not a hopeful line in a roadmap.

Build a thin internal layer before building a platform. A small app that routes approved documents to a model and records the answer may be enough. You do not need to recreate an entire vendor category to remove one expensive bottleneck. Start with one process, define failure handling, and measure usage before adding features.

Atlassian's total-cost guidance lists costs such as installation, integration, migration, licensing, infrastructure, and future vulnerability or upgrade risk. Those categories are useful even when the software is an AI wrapper. The custom code is only one line in the ownership bill.

What should you do this week to make the decision?

Pick one workflow with a named owner and write down the current monthly SaaS bill, manual hours, failure cost, and data restrictions. Ask the vendor for the price of the exact tier you would buy, not the price shown in a sales deck. Ask engineering for a build estimate that includes testing, monitoring, and six months of maintenance.

Run three cases in the sheet: expected usage, low usage, and high usage. Mark every number as a fact, an invoice, or an assumption. Then set a decision rule. For example, you might require build to break even within your chosen horizon and pass the security review before it can proceed. That is your rule, not a universal benchmark.

If the result is close, run a short proof with the bought product and a narrow internal prototype. Compare time to a correct answer, admin effort, failure recovery, and the work required to change a source document. The better option is the one your team can still operate after the first person who built it is busy elsewhere.

If you want a second pair of eyes on the spreadsheet or the workflow boundary, contact DeuceWorks. We can help you price the ownership you are currently leaving out.