I was thinking about this today...
It is true that price determines difficulty, not the other way around.
But I think it makes more sense to only look at continuous costs (like electricity) since you're looking at a barrier to entry with the mining rig set up.  You might take this into consideration for average cost of keeping a mining rig (meaning price/lifespan of the rig) but I wouldn't look at it for just those to start.  People should only drop out of mining if they could make more money selling their GPUs used than they would by continuing to use them (minus the cost of electricity)
So difficulty will rise to the point at which your average bitminer is indifferent between continuing to mine or sell the rig, basically.