i've used authorize.net and wells fargo (via Quickbooks). both are teh suck, but wells fargo's sucked less for its partial integration into quickbooks, and authorize.net sucked less for easily done batch transactions (this was for a non-adult business that required recurring billing).
i don't recall the exact fees, but it was something like 2-2.5% per transaction, 30 cent minimum. amex was around that. discover was 1% more (where do you think the "cash back bonus award" comes from, heh.) and had a monthly fee of like $15, IIRC.
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