What it actually costs to build an in-house ROI calculator
The build estimate is usually right and the total is usually wrong. A breakdown of the four costs teams leave out when they price building against buying.
Most teams estimate the build correctly and the total incorrectly. The first working version of an ROI calculator genuinely is a week of engineering. The costs that decide whether building was the right call arrive in years two and three, and almost none of them appear in the original comparison.
The estimate that gets made
A sales leader asks for an ROI calculator. Engineering scopes it: inputs, assumptions, a calculation, an output a rep can send. Two weeks, maybe three with a coding agent helping. Against a vendor quote, building looks obviously cheaper.
That estimate is usually accurate for what it describes. The problem is what it does not describe.
Cost 1: maintenance, which is the largest and the least visible
Pricing changes. A new product line ships. A driver that mattered last year stops mattering. Each of these requires someone to update the model, verify nothing downstream broke, and tell forty people the assumptions moved.
Budget this as a recurring claim on an engineer rather than a one-off. The work is small each time and never finishes. In practice it lands on whoever built it, as an interruption rather than planned work, which is also why it slips.
The failure mode is specific and common: the model goes six months without an update, reps notice the defaults are stale, and they start overriding them locally. At that point you are paying for a calculator and getting spreadsheets.
Cost 2: the support burden
Every internal tool has a person. Reps ask why a number looks wrong, request a driver the model does not have, or need the output in a format a customer asked for. These arrive as Slack messages rather than tickets and they rarely get counted.
The structural problem is concentration. The tool has exactly one person who understands it. When that person is on holiday, in a deal review, or no longer at the company, the support queue has no one in it. Bus factor one is tolerable for an internal dashboard and uncomfortable for something producing customer-facing financial claims.
Cost 3: the quarter it was quietly wrong
This is the cost nobody budgets and the one that actually hurts.
A homegrown calculator produces a plausible number from a stale assumption or a formula that broke when someone added a column. Nobody notices, because the output looks like it always looks. Cases go out. Eventually a customer's finance team checks the arithmetic and finds it before you do.
The direct cost is one damaged deal. The real cost is that your champion defended the number internally and was wrong, and champions who get burned stop carrying your numbers. There is no line item for this and it is frequently larger than the build.
Cost 4: what the engineer was not doing
The engineer who builds and then maintains the calculator is not building the product. For a company at Series B, that is the most expensive hour on the list, and it recurs.
Worth asking plainly: is maintaining a value model the thing you want a senior engineer spending a day a month on for three years? Sometimes the honest answer is yes, when the model genuinely is unusual. Often the answer, once stated out loud, is no.
What the comparison should include
| Line | Build | Buy |
|---|---|---|
| First working version | Weeks of engineering | Configuration, typically two to four weeks |
| Keeping the model current | Recurring engineering, indefinitely | Vendor's problem |
| Internal support | One named person, uncounted | Vendor's problem |
| Governance and audit trail | Build it or go without | Usually included |
| Post-sale retrieval at renewal | Rarely built, because it is not a problem on day one | Usually included |
| Risk of silent wrong output | Yours | Shared, and the vendor has more cases to catch it |
| Exit cost | Every case built so far | An export |
The row that most often changes the decision is post-sale retrieval. A calculator is built to produce a document today. Proving delivered value eighteen months later needs the original assumptions retrievable and comparable against what actually happened. That requirement is invisible when you scope the build and expensive to retrofit afterwards.
A fairer way to run the numbers
Compare three years, not one. Include the maintenance as recurring engineering time rather than a rounding error, name the person who owns support, and put a number on one damaged deal even if it is a guess. Then compare against the vendor's three-year cost including implementation.
Buying does not always win that comparison. For a small team with a stable, simple model it frequently loses. The point is that the comparison most teams run is not close to the real one, and it is biased in a consistent direction.
The question that settles it fastest
If the calculator broke tomorrow and the person who built it had left, who fixes it, and how long before the next deal has a business case?
If you can name the person and the answer is days, you have built a system and the economics may well favour continuing. If you cannot, the build was cheaper than buying only because the maintenance was never priced.
FAQ
What does it cost to build an in-house business case tool versus buying one?
The first version is usually two to three weeks of engineering, which is genuinely cheaper than most vendor contracts. The three-year total is the number that matters and it is dominated by maintenance, internal support, and the cost of periods where the tool is silently producing wrong figures. Teams that compare year-one build cost against year-one licence cost are comparing the two least decision-relevant numbers available.
What is the real cost of maintaining an in-house ROI calculator?
Plan for a recurring claim on engineering time whenever pricing, packaging or the value model changes, plus an uncounted support load that falls on one person. The amount is modest per event and it never stops. The practical risk is not the hours; it is that the work is unplanned, so it loses to roadmap priorities, and a calculator with stale assumptions is worse than no calculator because reps still use it.
Our RevOps team wants to build ROI calculators in Salesforce. What are we underestimating?
Usually three things. Governance, meaning who owns the defaults and whether overrides are visible to anyone. Retrieval, meaning whether the case built today can be reconstructed at renewal in eighteen months. And the reporting shape, meaning whether the data lands in a form that can answer which value arguments actually won. Each is buildable in Salesforce and each is substantially more work than the calculation itself.
Is a homegrown business case template good enough for enterprise deals?
For a small team with one person building every case, frequently yes. It stops being good enough at a predictable point: when more than one person produces cases, when a customer's finance team scrutinises the method rather than the total, or when the case has to be revisited at renewal. The template is not the weak part. Consistency across people and across time is.
How do we decide without guessing at the maintenance cost?
Look backwards rather than forwards. Find an internal tool your team built two or more years ago that is still in use, and ask its owner how much time it has taken since launch and how many times it broke without anyone noticing. That number is a far better estimate for this build than anything derived from the specification, and it is already sitting in your own organisation.
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