how much to tip on 'free' engagement shoot?

greg and i found an up-and-coming photographer on craig's list that's going to do a free engagement shoot for us in central park tomorrow. i figured i should offer him a tip but i really don't know how much? the shoot should last about an hour.... he may also bring a 2nd photographer... so if it's just him, what do you guys think i should tip? and if it's two of them, how much?

thanks for the input ladies!! xo

i think i'd probably give $50 to the main guy (regardless of whether he brings an assistant or not) and $30 to the assistant (if any)

i might give a bit more if he's also going to do your wedding photos