Each costs about $150K, does a static amount of heating/cooling/whatever, and is a one-shot. Then you also have the ship hardware that uses ship energy to do the same effect. These cost $450K (base, discounts available with friendly or allied planets) but they have a bunch of advantages. First of all they turn battery power (free at home, or at least dirt-cheap) into terraforming. Second, they can be used for major or minor adjustments, so you don't run into the problem of blowing a $150K one-shot and finding that it didn't quite get you to T2... and now you need to go back and buy two of them. In other words, the $450K price tag is exactly what you would spend on a single botched terraforming run.
The ship add-ons are unlocked by doing environmental missions, but I think you can get there by just bringing wildlife to a bunch of new planets. Once you get the add-ons, the game changes drastically - for an all-green player (peaceful/shaman) you can spend your whole energy reserve on terraforming a planet, then "hearthstone" home and recharge for free... which means you can make all of your colonies T3 with maxed-out turrets. The colony upgrades are still expensive, but the terraforming is essentially free.
TL;DR - Buy the ship addons, and don't waste your money on one-shots!