Depends where you live?
I'd expect to pay about a fiver a board, I'm a hagler too so if I think there's a chance of getting a bit knocked off i'll have a go. It's a case of shopping around you might be able to do a deal for 80 boards. I have seen someone suppling them on ebay too with free deliver but most builders merchants will deliver free over local distances if you spend over £300. Best advise shop around but don't forget the fact that your time and travel comes at a cost also.