It took me a while to figure this out too and only did in the last couple days: a 10x, 20x, or 25x plan is the cost of your monthly sub times that multiplier in terms of API costs.

So a 20x plan, that was formerly $200, would have been $4000 in API credits. Which sounds like a lot but the API is absolutely horrendously expensive - like you'll burn through $100 in 45 minutes of 5.6 Sol on High reasoning. I know this because I did this once.