Well they charge per prompt, but with usage limits it is a mix of token and prompt. If prompt multiplier is higher, tokens are also multiplied, so limit is reached sooner.

It is basically a token based pricing, but you get alos a limitation of prompts (you can't just randomly ask questions to models, you have to optimize to make them do the most work for e.g. hour(s) without you replying - or ask them to use the question tool).