I found this guy in the UT honors program for computer science who wants to help me build my two websites but i was wondering...regarding payments, how should i go about paying him? half now, half later?

should i ask him if he'd be willing to receive payments on it after finishing and it earns profits?

ALSO, how much does it cost to develop two complicated websites anyways? I'm talking like facebook or ebay...he will be designing the guts of the site. how much should i offer him since he's a college student?