I used to book bands. Folks might be surprised at how little you would really have to fork over to get a decent celeb for a private party.
Especially if you are talking about a 2nd tier guy or an older dude, who hasn't done much in awhile.
Say, someone like Susan Vega. I know that she plays a local coffee house around here...I can't imagine she gets more than $2000 - $5000 for a show.
If you grab a gang of a few guys, called up William Morris or one of the other big agencies...you might be surprised who you could book for a private party...
Most the musicians I know are more studio players, people who have toured with folks like Roger Daltry, that kind of thing...those folks you can sometimes get for even $1000... I used to book a band of guys like that...and their take for the night was under $500.
So, anyway, if you want to give a friend a killer birthday....(don't contact me about bands...I don't do that anymore....just saying...it's a cool idea to have some fun).