It's hard to find free plugins that integrate payments...if you're making money, you ought to be able to pay for a plugin, I guess. Also, this kind of functionality is complex, a lot of time is involved in getting it to work...and it's a critical application, due to the fact that money is involved.
My suggestion to you is to handle it manually. Find a good place to put your PayPal payment button (like on the signup thanks page), then manually update each registrant's record when they pay.