Marine Plywood is a very generic term these days.....I agree, the good stuff will be over $100 at a local lumber yard....I think I paid $140 for a sheet locally that I needed bad and couldn't wait....but if you need a bunch of sheets like 6 or 8 and you need some foam sheets like Coosa or Divinycell and you get it from the same place, a pallet of goodies breaks down to under $100...but you do have to shop around..
As far as the resin? Well, either I'm getting to be a worry wort in my older years or you and NEM are getting cheap, I mean frugal in your years.....
After removing the core on a Sailfish, fixing dings and divots on the inside of the outer skin, laying in a layer of 1708 to replace the material lost during prep work, taping the perimeter to replace some cheap ass polyester putty, then laying in 3 layers of full width ply and end filler strips, all with pre-priming of the plywood with neat epoxy (2 coats) and using a notched trowel to lay up the thickened epoxy, I used 5.5 gallons of resin.....then comes the 4 or 5 layers and tape for the new inner skin of 1708 and 1810.....any other work like battery trays, tying in the stringers, misc fairing putty etc and you'll be in the 10-12 gallon range.....
The notched or cut out transom will give a little reprieve in terms of material, but I don't think it will cut it in half...maybe if you use coosa which doesn't absorb much resin, it will be a little less......but I definitely come form the school of thought that the extra resin will only help, not hurt me......other than my wallet...