you will need no more than 3 cartridges,2 should be plenty,if you pre-fit all your metal perfect, but if you wanted to build up the sides,it may take a half of one cartridge, and where in the world are you getting those prices? I buy it for $25 a piece, check Sherwin Williams or Albert Kemperly,they have over 30 stores nationwide,they might have one near you,
Stefan