You certainly can. But it involves a lot of risk too, since there is no guarantee of those profits in the future.
In the peak of my GPU mining, I could make $100/day. Now, I am making $6/day, and more than half of that goes to pay the electricity for them. And that return will get even lower as more of these more efficient miners start being used.
Dude! When were you making $100/day?
Middle of June IIRC. It was when the price hit its peak of $31... difficulty was still low enough that 1.8 GH/s was making me 3-4 BTC/day.