The guy that made my steel wheels (for my 68 GTS) charged $95 for 15X7 and $100 for 15X8 + actual shipping. Since he uses a donor SBP center and mates (weld and balances) it to a 15" hoop, he can make whatever offset you need. I believe mine were 4.5" offset.
He's very knowledgable and I found he was almost finishing my thoughts as I rambled on about my "needs" during our phone conversation. He's been doing this type of work a long time and actually makes wheels for the previously mentioned distributors.
I doubt you'll find any better deal @ the big box car stores/on-line. It was also nice to talk to this small business owner and help him keep his employees busy and doors open.
FWIW, I was after the stock look (dog dish/poverty caps etc.), not tire selection. I had seen another GTS on this site with the 15's filling up the wheel wells and like the look.... plus it puts more rubber under me as I have the big T-bars, F&R Hellwigs and KYBs underneath.
PM me if you'd like additional contact info.
Pete