I pre-ordered the Alien Anthology Blu-ray set when it was first announced and even though I paid more than the discs are now selling for I don't regret it one bit. Prior to any reductions I'd already spent hours and hours pouring over the special features and watching the different versions of the films on the discs.
The Alien Anthology Blu-ray box- set replaced the DVD box-set that I already owned and I was hugely impressed by the difference in the transfers. I've even actually used the Blu-ray and DVDs to convince skeptical friends and family members about Blu-rays. The transfers really are startling good in places.
There are a lot of other reasons that I could offer to convince you that it's a set worth grabbing, I haven't even mentioned how good most of the films are, but their current price is probably enough to sway anyone even vaguely considering it.
Amazon currently have the Blu-ray 6 disc set for just $19.99, which is 71% off their list price and just $3.33 a disc.
If you're in the UK you can currently get the set for £18.00 which is also very cheap, working out at only £2.00 a disc.