Don't pay for it. When Ground Control 2 came out they released the first one for free as a way to advertise. I'll search and see if I can find links that still work. I remember downloading it through fileplanet way back.
EDIT:
http://www.fileplanet.com/promotions/groundcontrol/Also, as for the game itself, I recommend it. It's enjoyable. I like the story much more than I liked Ground Control 2's, because they made some goofy aliens you can play as for GC2 while GC1 is humans vs humans.
The story in GC1 is that there are archaeological artifacts on the surface of an uninhabited planet. A giant space corporation and a crazy space religion send armies to fight over them. Story develops between the individual commanders.
Some things to note is that infantry comes in a squad for GC1 instead of individual soldiers if I remember correctly. Also, the artillery pieces are really fun to use and watch arc to their target. Artillery battles are fun. I never played multiplayer but that would be fun to keep your artillery moving while trying to counterfire his artillery by guessing from trajectory where it's coming from while advancing an army that would otherwise be bombarded. In the single player campaign your units are extremely precious and losing a single one could be a cause for restarting a mission. This is because they carry over from previous missions, earn experience, and the game keeps track of their kills and such if I remember correctly.
If you enjoy GC1, then you'll probably enjoy GC2's gameplay even more. I didn't like the story quite as much since I enjoyed being able to relate to both sides in GC1 along with it's believable simplicity. GC2's story is more: evil human empire invades your home, they have alien slave soldiers half of which defects to your side. The units are much better done in GC2, so gameplay wise it is superior to GC1. The 3 sides you can play all have interesting units and the aliens play slightly differently. There is a "dropship" resupply system so your unit's are not as precious as GC1. You spend points which accumulate over time on new units, which the dropship (which is armed and can be ordered to fly around the map before leaving) will land in a landing zone. Controlling these landing zones generates the resupply points as well as allowing hot-drops from an armed dropship. The dropship also can be upgraded with resupply points. The upgrades include capacity, weapons, speed, shields, and fuel supply (which determines how long the dropship can provide air support.) Overall it's a very interesting system that impressed me. Also there are a large variety of strategic fixed emplacements that your troops can occupy once taken and repaired. Also, units have an alternate fire mode. For example, the basic infantry has an automatic weapon and also a small guided missile launcher for use against vehicles. Infantry can also garrison inside buildings facing in the direction you tell them to.
Also, both games feature various infantry, personnel carriers, aircraft, vehicles, AA vehicles, and artillery, a good pathfinding and targeting AI and the campaigns are challenging, although for slightly different reasons. (GC1 = restart mission if you screw up badly while GC2 = the possibility of extremely long missions building up forces to take a position with emplacements while the computer sends waves of troops at you {this isn't neccessarily a bad thing. It can be fun if you find a defensive position, there is one mission in particular that you can set up snipers in a building and scout cars in a line with their long range cannons supported by tanks and infantry to plink down advancing enemy waves before they can pose a threat which is pretty fun to get right.})