Black Mesa is Half-Life that comes back with a vengeance. I's say it's the FPS equivalent of Hatchetman coming back from the grave in a shiny semi-new body. I've finished it recently, and can't wait till they do the Xen part. Also - the writing and new additions/refits of the story and maps are great. My favourite quote: Security Guard at the Lab entrance to Gordon "Man, you cut the ponytail! Sellout." That's corporate life for you.

As for the price - you can download the Half-Life 2 Source for free on Steam. Then you just download and install Black Mesa and play... It's really back. Get ready for Unforseen Consequences.
P.S. The soundtrack is perfect for a Shadowrun game too!