I enquired about The Beach Boys a few months ago.
For two men (only one an original Beach Boy) and a small inconsequential backing band, they wanted full expenses (first class flight from California and 5 star hotel, hire of sound stage and roadies) and �250,000 up front. So I guess all in about �300,000.
For �20,000 you will get Steve Harley if you're lucky or maybe a new comer like Ray Quinn.
However, I would opt for the Brighton Beach Boys. They sound like the real thing and would make any ball a very good do as they play a myriad of different styles. I reckon they will put on a show for about �8,000 so you can spend 12k on beer!!!! (or anyother band)
http://www.brightonbeachboys.com/index.html