I'm going to simplify it a bit. Visa doesn't actually authorize
Visa (issuing institutions do via authorization institutions), but
let's pretend they do. They charge a fee to authorize that
transaction. They charge that fee to the settlement institution.
The fee varies depending on the risk level. Visa (or whomever) has
some well defined rules for fee/risk. The setttlement institution
passes that fee onto the merchant by reducing the funds that they
receive. It is up to the merchant to absorb this fee. In fact,
most major credit card companies have explicit rules against
charging the buyer extra to cover the fees. To my knowledge, they
do not have rules against charging the seller.
This is where PayPal makes it's money. They charge the seller for
these authorization and settlement fees, plus a little on the top
for themselves. They also make money on the financial float (the
period of time they hold the funds).
Having said all that, as Marty said, PayPal lays it out here:
http://www.paypal.com/cgi-bin/webscr?cmd=p/gen/fees-receiving-outside
P.S. There are several variations of how this works. I wasn't
going to write the book it would take to cover them all. If you're
a merchant, you might see it a little differently.