The value of 1 ad credit is ~2500 R$ and is actively adjusting. However, the value for 1 Ad credit as listed on the documentation is currently 285 R$.
Page URL: https://create.roblox.com/docs/production/promotion/ads-manager
It’s 285 robux per 1 ad credit. That’s for how much I bought them recently.
My mistake, I read my robux amount next to the Ad credit purchase, sorry!