Some actual numbers may be of help.... a typical bulb might last 2000 hours, and typically, cost £250 As to distances, and from there directly to image size, check out
http://www.projectorcentral.com/ It's a yank site, but much info especially projector specs, and distance/size calculator, and price quotes. They seem to translate well into pounds, in real terms we pay roughly the same, that is after you've beaten up the dealer with 3 other quotes of course.