I was going to post that Stardock can likely sell 20-50 times fewer copies and still make a very tidy profit, as spindoctor did.
I was going to question it, though, as others have. Trust me, that's a huge chunk of this logic, but it shouldn't be easily accepted.
I do NOT think DRM is having major effects on sales. Maybe Ubisofts new DRM does, but by and large DRM isn't preventing a significant, or even note-worthy, chunk of sales.
The question is whether it's causing any sales by preventing piracy. Again, I think down the road it will. I think uncrackable DRM is coming. How? Clouds. We'll all need to hook into a cloud via the internet to play a game, similar to what Ubisoft is doing now (several years too early.)
Until then I don't think it's doing a damn thing. There's no way to tell how many pirates would buy a game if they couldn't pirate, but I see no evidence to assume it's a sizeable amount. Beyond that, making a game harder to pirate doesn't discourage anyone. If it can be done it'll be done. No one will look at a game that requires 40 steps to pirate and go buy it. The people pirating are sophisticated enough to not be scared away (by all this I mean those downloading the game, not those cracking and releasing the game.)
Your set of pirates and your set of purchasers probably don't overlap as much as people think. It's not like used game sales...