The oppertunity cost is ~$0. It can be merged mined right along side Bitcoin. The hardware, labor and electricity would still be spent if one was only mining Bitcoin. Nobody is mining this at a loss it is just a free bonus on top of the BTC.
To be fair, while it is low, it is not zero. Running it takes up cpu and memory and network resources etc.
Edit:
Ah, just noted that you said opportunity cost not cost... ok... that would be zero when mined with bitcoin, and other mm coins, and be part of the cost of switching from them.