Can you?

I am pretty sure that the rules about IAP make it very clear you have to charge the same price on and off the app store.

No it doesn’t. In fact, when Spotify was still allowing in app purchases, they did exactly this.

https://www.theverge.com/2015/7/8/8913105/spotify-apple-app-...

That was Apple's initial attempt, but the pushback was fast and furious. Spotify, especially, led the charge that this was ridiculous and Apple relented pretty quickly, IIRC. Now you can charge more to cover their 30%.

OK, that's probably what I remember then