We get asked all the time to work on a "pay for performance" basis.
Some companies ask because they have no money, and others ask because they've been burned before. But both typically try to use our claims of competence as justification for not paying (i.e. "If you're so good...")
Both types also assert that, if we can get them in the door, they can close the deal. And as a result, they say, we're sure to get paid on the back end.
The Rest of the Story...
The funny thing about being so confident in your ability to close, though, is that if your salespeople were so good at closing, they'd also be good at prospecting.