30% is ridiculous for Apple and Google. It's an arbitrary number that they could get away with when the Appstore came out nine years ago and everyone ran with it ever since.
I don't know what a fair amount is. But I doubt it's in the two digits.
You just have one large customer in one country instead of thousands/millions in many different jurisdictions each with its own tax and consumer protection laws. That's worth more than 2.5% but it's not worth 30%.
I would say fair is somewhere between 5% and 10%, considering they also provide some distribution infrastructure on top of payments, billing, taxes and compliance.
Well, I guess Symbian got N-Gage and the Ovi Store towards the end, but they never tried to lock it down against third party sources.
When Apple announced 30% cut, mobile devs where cheering, because many providers were charging way above 50%.