Is it normal to pay to third party processors 15-20% of the turnover?

And chargeback expenses also lay on you! Also as far as I understood they don't deal with off-shore companies. Please help me to clarify the situation, why it is so expensive and does every adult site pay something around these numbers? Lets say I run live cams and have a turnover of $50,000 weekly, quite big sums for charging. I tried to plan live cam site and can tell that it is not possible to succeed paying 20% for charging + taxes if it is not an off-shore biz.