it was originally going to be just GMod, but we went to $12 in about 20 minutes, so I decided to give ducky CS:S as well since I didnt have to work out who to give money back to!
I will definetely have to do stuff like this again, it worked really quite well.
As per Psy-T's comment, he can explain the original concept, since he was involved with that, not me.