Mechcommander 2 is freeware.
Microsoft released the sourcecode years ago and there are now at least 2 fan made stand alone expansions available for it. The first is
Wolman-X's MC2-X and the other is Magic's
Omnitech.
Wolfman-X is older, more polished, and truer to the original game. Omnitech is less polished but has added some really interesting touches to the game, specifically in regards to managing your units' load-outs and available equipment.
Both versions include the toolset for making custom campaigns and missions, and many fan-made campaigns are available. MC2X includes a "brain tool" that will allow non-coders to program enemy AI.
The only downside is that these fan made versions don't support online PvP or in-game videos, so I would still love to see GoG sell the original retail version.