But if GPU mining is relatively cheap, more GPU owners will jump on it, making the difficulty higher, giving less reward, removing the relative cheapness from GPU mining.
Higher difficulty and less reward per CPU core gets irrelevant when nobody mines with CPU. If GPU mining is 25x more efficient (as was claimed somewhere in this thread), structure of the mining public will change. No more few giant miners with big expensive rented VPS farms, instead large majority of smaller miners with already owned GPU cards with only electricity costs to cover. Wouldn't that make mining cheaper, allowing everybody to sell XPM cheaper? This seams obvious to me that GPU mining will drop the XPM price significantly, maybe I'm missing some factors.