Chris, this will be a rough guess but is based on my rebuild. This assumes you remove the inner skin of the transom and chisel out the existing core. Then, cutting the plywood to recore and fill in the open area. Gluing in the new core, tabbing it to the hull, then building up the new inner skin.
Three sheets of 1/2" marine ply (or 2 sheets of 3/4). About three gallons of epoxy resin. Five yards of 1700 Biax cloth. Thickener (woodflour/cabosil) fairing compound, primer, & paint.
About $550 for supplies.
20 hours of labor. (+/-)
Hope this helps.
MD