Market price in this case is retail. Its the price that will move the inventory of lobsters for the most money, before they die of starvation or kill/eat each other in the tank.
Of course lobster pounds and the like have a market price that's usually posted on a chalk board or whiteboard.
What I was referring to was regular restaurants that routinely don't list a price for dishes that use lobster on their menu and instead just say "market price." Which, in my experience, is more or less a synonym for "expensive."
I'd note that, in addition to lobster pounds, it's far more common for coastal Maine restaurants to have the price for lobster in their menu like they do for every other dish.
(It's fair that lobster pricing does fluctuate more than other proteins but "market price" is at least as much about reducing pricing transparency.)
So, the short answer is that lower lobster pricing doesn't seem to have led to widespread lower pricing in most places.