1. You shouldn't have agreed to do the gig for $300 to begin with. If these folks have dough then they probably wouldn't have even minded paying full price for the band. (My wife and I do fine money wise. I recently had one of my best friend's bands play for me. I wouldn't even think of taking a discount because I knew I was getting the full product, and not a reduced form of the product.)
2. Being that you DID take the gig, in my view, it was pretty much a freebie anyway. You were doing them a favor. So put out the jar and go for it. I have been to private parties where there was a jar and tipped myself. I wasn't offended at all.
3. Next time, stick with #1 and don't take a $300 private party. That isn't a discount. That is an insult. I wouldn't play a club gig for that, much less a private party for a rich group. Tell your guitar player the next time he wants to do a favor he can pony up the difference in what you would have been payed otherwise.
By the way, before any of you jump, a fund raiser is a totally different story. I play them all the time and really enjoy donating my time. Everybody else pays full price.... no exceptions.
+100