i'm not sure how helpful my suggestions will be but here goes...
I agree with jeremy, if a hacker really wants to get that prize they will but it also depends on how much the cash prize is (or whatever the value of the item). If it's something small valued less than $100 you shouldn't worry too much about it, just get some encryption happening.
Otherwise I really think a combination of PHP SSL & Cookies will get you a good amount of security, as I know a lot of shopping cart systems use SSL to handshake with the browser to prevent hackers from stealling information.
Finally if it's a really big some of $$$ then i would most likely look into another technology, shockwave maybe?