There’s an updated (May 2023) Power Platform Licensing Guide which adds tiered pricing for the AI Builder credit packs. The packs include 1,000,000 AI Builder credits per month and if you buy under 10 packs they are $500 each, if you buy 10-49 packs they are $375 each, and for 50 or more packs, they are $250 each.

Find the details on page 20, and get the updated guide here: https://bit.ly/3LrdNMJ.