I'm sure it's been mentioned before, the Alien Anthology (all 4 films, 2 versions of each) is £14.99 at Amazon. Just bought it to refresh my memory before Prometheus.
EDIT: Just read the old thread, and it's the first post! Doh!
Make sure you get the right one - greenish cover - I think there's a newer one that is 4 discs not 6. The older one is a classic - and surely there's no better Blu Ray bargain out there.