Transom Shops In the Boston to Cape Cod area?

[erebus]

Approx $750 total in materials for a 4x8 sheet of composite core (about $350), epoxy (about $300) and glass material (about $100) with about 30 hours of labor... hmm, sounds about right.
What does he propose finishing the outside with?
Spraying awlgrip I'd add another 3-500 to that quote.
A quick roll-on of gelcoat would be in your ballpark.
Labor could be higher overall depending on how easy/hard it is to get the old core out (i.e. falls out or needs to be ground out).

If he's using straight up marine plywood, and cheap polyester resin (I don't recommend it) than it could be a bit cheaper.