What's the guarantee that terriforming is economical, controllable, successful, sustainable, or permanent? I think the idea is taken too much for granted. It's easier to destroy a climate than to make one.
If a civilization is able to terraform a planet then the civilization would already have the economical aspects of the terraforming project down to a science.
^ What are we talking about? Hospitable climates, or just any old climate, including hostile ones? I'll award you a point for being right on an effing technically.
But evolution took centuries to create the concept of economy, which not all species share. Granted, if the spider in the basement asks me for change for a $10 I'd try to help him out with six $1 bills and two $2 bills... 'course, he'll want the $10 back since now all his eight legs are holding something when before only one was holding something (so economy isn't always about $, hehe...)