Custom build or monthly subscription? Run the three-year math.

Subscription AI tools run $15–$500 a month for small teams and $1,200–$5,000 a month at enterprise tier. A one-time build is a larger number on day one and frequently a smaller number by month eighteen. Here is how to work out which side of that line you are on.

The arithmetic, done plainly

Take the subscription you are considering and multiply by 36. A $300-a-month tool is $10,800 over three years. A $1,500-a-month enterprise seat is $54,000. Now compare that against a one-time build plus realistic maintenance at 15–20% a year. The crossover usually lands somewhere between month twelve and month twenty-four — earlier if you have more than a handful of users, because most SaaS prices per seat and builds do not.

That is the whole calculation. What it misses is the part that actually matters.

The question the maths does not answer

Is this workflow generic or is it yours?

If what you need is the same thing ten thousand other businesses need — a support inbox summariser, a meeting notetaker, a general writing assistant — buy the subscription. Someone has already built it better than a bespoke version would be, and they will keep improving it while you sleep. Building generic software is how companies waste money.

If the workflow is specific to how you operate — your rules, your edge cases, your systems, the exceptions your team knows by heart — a generic tool will get you 70% of the way and then stop, permanently. The last 30% is where the value was.

The costs that do not appear on either price list

• Switching cost. A subscription you outgrow costs you a migration. A build you own costs you a decision.
• Data location. With SaaS your operational data sits in someone else's system, shaped by their model. That matters more in regulated work than anywhere else.
• Price control. Subscription pricing changes at the vendor's discretion, and AI pricing has been volatile in both directions.
• Compounding. A build you own accumulates your rules and your corrections. A subscription resets to everyone's average.

The honest recommendation

Most businesses should do both, deliberately: subscribe for the generic layer, build the one workflow that is genuinely yours. The mistake is not choosing wrong — it is choosing without doing the 36× multiplication first.