Do you need it for adult transactions or for mainstream?
If you need it for mainstream Paypayl has a micropayment plan, look here:
https://www.paypal.com/IntegrationCe...opayments.html
Quoting directly from Paypal about pricing:
Micropayments fees for US dollar transactions are 5% + 5 cents per transaction. When compared to PayPal’s standard fees (2.9% + 30 cents), the micropayments rate can save money for merchants whose average selling price is below $12.
End quote.