I'm guessing but a slight premium over 1F 45 ACP I should think, say about $0.12 apiece. I know its less common and will be even more so when they quit making it but if its just trimmed down 45ACP brass then the price for GAP brass wouldn't command a big premium IMHO. You can get 1F 45 ACP for a dime.
45 GAP is higher pressure than 45 ACP and has thicker case walls. You can't make real 45 GAP brass by trimming 45 ACP.