I've only ever seen the first one and really enjoyed it. Nice slasher movie with one of the most "interesting" twist endings. From what I've read, the sequels are more tongue in cheek but still entertaining as they have lots of gratuitous nudity in them, just like most of those old horror movies you would rent from the video shop on VHS in the 80's.
1p more at play.com (so if you have a voucher or summat go there) and quite a bit more everywhere else. The cheapest price for Sleepaway Camp 1 on it's own is £5.49 so this is a bit of a bargain, and remember, it's worth it just for that ending in the first movie.