It would not be profitable to build a dedicated rig?
Hard to say. Too many variables to guess.
What if i just added a second card to a computer i already have? Would that be worth it?
Depends on how cheap you can get it and what your electric costs are. It would probably pay for itself if you can get a decent price for it, but unless you have free electric and AC or are heating with electro-resistive heat then it's almost certainly not going to cover it's own AC & electric costs.