Costs regarding removing closets in a coop?

If the closets are big enough and located so that the combined space would meet the building code requirements for a proper room (there are window and egress requirements), then sure, you could do it. Make sure you know what's lurking behind the walls that would be coming out, otherwise it may be impossible.

My 1BR had a good sized (5' square) walk in closet in the front hall that I configured as an office -- I needed workspace more than the add'l closet space -- a built-in desk along one wall, a built-in bookcase along another wall. As a work cubicle it's ok (better than some I've worked in), and I can close the door and guests don't have to see a messy desk, and I don't have to clean up.