Have a look at Crypto's conditions. They've made a big deal about how rakeback is the same for everyone, and it's actually less than it once was.

50% is the rate for people earning 11k MPPs a month - playing $100NL you'd have to get through approx 50,000 hands to reach this level. On top of that, net rb is actually something like 38% - Crypto take 22.5% of the rakeback as some kind of admin charge.

If I am wrong, perhaps Sun's nice rep who posts here can explain how it works.