I have a friend that just put in a very large, #10,000 granite block in my area (Portsmouth, NH) in deep water. His cost for all materials, installed was in that ballpark. I think that your quoted price might be a little high for a mooring that small, but not terribly high. A #5,000 concrete block runs around $5,000 here, then there's also chain, mooring, and labor.
Consider calling around to different service companies to get competing quotes.
If you know a diver that can run lift bags, you may be able to get a serious discount by hiring him/her to do the labor and working directly with a concrete/precast concrete plant for the block. Plants will make waste blocks all the time with excess concrete. Anything they get for the block is return on what is already waste. You might be able to work a deal out with the plant to throw a steel loop in the waste block for a connection point, which is common practice anyway since they need to move it around somehow.
Moving the block and launching it could take some creativity. Around here (8 FT tides), a local service would launch the block at low tide, then float it at high tide, tow it, and descend it with divers and lift bags.