I thought Mars was a pq4 so you could inhabit it. There are a couple problems with terraforming planets.
First, you have to create an atmosphere that supports life, which on a barren rock would most likely require to BYOA (bring your own atmosphere). On larger worlds this would most likely require tons of material. The average human breaths about 11000 liters, 388 cubic feet, of air in a day. This includes the other gases as well, i.e. nitrogen. If you have 5 billion people on earth that is roughly 2 trillion cubic feet of air used in a single day. This is just atmospheric needs as you would need fertile ground for the planet which would be 148 million square km for a planet the size of earth. I am not sure how much dirt that would require to support life but I am betting it is a lot. So would you rather spend trillions of creds on terraforming a planet or travel a few weeks away and colonize a pq10+.
Second, so you can get cheap air, maybe from sucking from a enemy race's planet, there becomes the problem of balance. Mother earth has spent the last 4 billion years balancing this planet. It isn't as easy as everyone makes it out to balance the ecosystem of a 5.972e24 kg ball of rock. Everything has to be balanced, and when you throw in a few billion colonists that have to be balanced.
Third, so you stole some poor planets atmosphere and you have cleaned out the equipment because it accidentally sucked up some hapless sky divers. What’s next? Maintenance. Some of these planets are lifeless balls of rock for a reason. If they are to far away from the sun you have no heat, or so little that it doesn't matter. This means that once you have colonized a planet there would be a race of how they would die from either freezing to death or starving to death as the local ecosystem died from a ice age everyone should have seen coming. In this day and age i guess you could create a machine to heat the planet, and provide enough light for you plants to survive, but i think heating an entire planet would probably not be cost effective.
Fourth, Lets say you have done all the above in a cost effective way, i.e. got a research victory ascended and used you vast knowledge of the universe, you still have to get the material from somewhere. I am thinking that removing that much mass from any planet may throw the gravity for that particular solar system off. So what happens with the planet you sucked all that mass from and the solar system it is in? I don't think it would be a good thing. Things in this universe tend to be connected, and i am sure it would come back and bite you on the behind.
In the end it is much easier to conquer your enemy’s planets, or find better ones, then to terraform. When it comes to just building habitats, that might be more cost effective, but nothing is better then free, i.e. already has a good atmosphere.