I would be offended if a friend of mine (or anybody) offered me half market value for any of my snakes. I might give them a deal because they are my friend, but if they asked for 50% market, I'd be annoyed.
I think it's reasonable to offer a lower price on a group purchase, and there is the benefit of not needing to ship, but I would still offer a fair value. Think of what's appropriate to offer at shows, and think if that offer to a stranger would be offensive. Don't be rude to a friend. Taking into account the quality of the animals themselves, their ages/breeding potential, and their worth to you.
Check out prices other Canadian breeders have for a reference, kingsnake.com, fauna-classifieds, and factor in differences with USA vs Canadian prices, and make a judgement call.