Why do some AI tools charge per month and others charge per use?

It comes down to how much computing power you're actually using. Monthly subscriptions are like a gym membership — the company bets you won't use it enough to cost them more than you pay. Per-use pricing is like paying for gas — you pay for exactly what you burn. When you use an AI tool, it's running on expensive hardware. We're talking specialized chips that cost thousands of dollars and suck up enormous amounts of electricity. A single complex request to a large language model might cost the provider a few cents in computing time. Ten thousand users making a hundred requests each? That math adds up fast. Take ChatGPT Plus as an example. For $20 a month, OpenAI gives you a set number of messages. They've calculated that most people won't exceed a certain threshold, so the subscription price covers their average costs plus profit. But if you're a developer using their API, you pay per token — basically per word or piece of a word you send and receive. That's because developers might blast through millions of tokens in an afternoon. A flat monthly fee would lose them money. Here's what I tell people trying to decide: if you're using AI for casual questions and occasional help, a monthly plan is usually simpler and cheaper. If you're building an app that will make thousands of calls, per-use pricing gives you control. You can cap your spending. You can't accidentally run up a bill on a monthly plan, but you might pay for capacity you never touch. One more thing — some tools mix both. They'll have a monthly base fee for access, then charge extra if you go over a usage limit. Always check the fine print on what counts as a "use."