Ok difficulty and exchange price.
But these are two independent variable or not? this is the question at the end, in order to make a comparsion.
They are loosely coupled, but difficulty moves much slower than the price.
Difficulty should follow the price, but with delay and limited change.
In basic sense price drives difficulty to state where costs of mining is near the produced coins' value... That is if price increases, more is invested to mining and thus difficulty will eventually rise to match it. Same should work in reverse if price where to drop in future...