"Difficulty" is a measure of how competitively the coins are bid for in the primary market.

But Dash doesn't expose all of its supply to that bidding process. Only about 45% of it is. The rest is supplied to existing holders at zero price. Those coins are then sold in the secondary market (exchanges) where the seller will be at a profit at any price all the way to zero, unlike their mining counterparts. That's what results in a decoupling (as far as that portion of the supply goes) between mined price ("difficulty") and market price.

The masternode is not penalised for selling below market price (they would be if the reward appropriately reflected their operating cost). The miner is however.