"For us, Black Mesa is purely a labor of love," the team writes. "We believe this philosophy has significantly contributed to the overall quality and feel of the game. Our decision to sell Black Mesa rests on two key points. One is we believe we can make the game even better by having full access to the Source engine. This lets us tackle and fix limitations instead of working around them. The second is because frankly, our team could really use the financial help."
The paid version will include a few features not in the free game, but soon after the paid version launches on Steam, a brand new free version will go live, too.
Black Mesa was one of the first 10 games to make it through Greenlight in September 2012, granting it a spot on Steam. The current, free edition of Black Mesa is available now through a number of hosts, including Mod DB. Check out the complete list under "Download" here.