It, like the other cross-world sorties at the outbreaks, is just a justification for its existence.
It has nothing to do with the cost and it never should be. You send in your best asset to do that kind of work. Yes a B-52H can also do the mission, but it has a radar cross section the size of a barn door and consequently it is at risk. Libyan air defences might be old, but anyone can get a lucky shot off. Why give the enemy any chance to down and capture Aircrew?
This is no different to any other conflict that the US has been involved in. The B-2 was even used in Afghanistan in 2001 to take out the air assets and old Soviet era SAM and radar networks still in Taliban hands. It is designed to be used and not sidelined simply to save money. Let the professionals get on with their job rather than worrying about cost.