How much does it cost to add AI to my business's website?

Three weeks ago a client who sells custom-made furniture asked me for a quote on an assistant that would answer questions about timelines and materials before the visitor reached the contact form. I gave him the number of development hours and he looked shocked, but not for the reason I expected. It was because I’d just explained that was only the first invoice.

Here’s the mess almost nobody explains upfront: building an AI assistant has a fixed cost (building it, connecting it to your site, training it on your catalog) and a variable cost that depends on how much it’s used, month after month, indefinitely. No expiry date. You pay the fixed cost once and that’s it. The variable one is what catches people off guard.

That variable cost is what the model provider charges — doesn’t matter if it’s OpenAI, Anthropic or Google — per conversation: it’s measured in tokens, roughly chunks of a word, and the more questions your assistant answers and the longer the replies, the more tokens it burns through. For a bot handling twenty queries a day, the spend is trivial, a couple of euros a month. For one that suddenly gets traffic from a campaign and handles two thousand conversations a day, the bill can multiply by forty without anyone touching a single line of code.

It happened to us with a home renovation client. We launched their assistant on a low-traffic site, budgeted a monthly figure with some margin, and three weeks in a social media campaign brought ten times the usual visits. The assistant worked perfectly. The AI provider’s bill spiked. And the client called us worried, thinking something had broken, when in fact everything was working better than expected, which is honestly a good problem to have. Nobody had warned him that scenario existed, though.

Since then we do two things on every project like this. We set spending caps directly in the provider’s dashboard, so it can never spike without anyone noticing. And we pick the model based on what the assistant actually needs to do, not the most powerful one by default: answering “how long does delivery take?” doesn’t require the priciest model on the market. A cheaper, faster one is more than enough, and that shows up clearly at the end of the month.

What I still haven’t fully figured out is how to explain this in the sales meeting without it sounding like “this could get very expensive and we don’t know how much.” Because that’s not really it — with the caps set properly the risk is manageable, it’s just a matter of configuring it beforehand instead of after the scare. But it is true that any fixed quote someone gives you for an AI project with variable usage is, at minimum, incomplete if it doesn’t also talk about that second invoice.