So yes, it is easier to build a settlement than to terraform, but I don't think you could justify the cost to build a self-sufficient settlement. Heck, if we start running out of land to settle on on Earth, there's always the sea to settle, which requires many of the same engineering considerations to settle but at much lower cost to get us there and do trade with the rest of the planet. I suspect we'll be seeing either underwater or raft-based settlements long before we see settlements on Mars.
(I feel like I should be using terms like "minimum viable settlement" around here!)
While I think that we're still a good ways from pushing this planet to it's limit (at assuming renewable energies come online quickly enough to mitigate population growth), eventually we'll want more room. And living in contained units (underwater, in space, or on other planets, ) doesn't seem like a worthwhile solution. So we're either talking population control or making more livable space. Surely, a thousand years into the future, we don't want to only have one livable planet to our race. And (maybe I'm wrong but,) terraforming what we have seems more likely that traveling to other Earth-like planets.
I'm not crazy enough to think that this is something we can overcome easily, or during my lifetime, but I think it's more of a "if we don't get started now, when?" type issue.