I have seen it yesterday for 5 bucks US a round at a gunshow. also saw it at 4 bucks US. That is considered decent pricing, 7 bucks US is considered HIGH pricing, I would say 7 to 10 bucks cdn would be more than fair, especially if he wants to sell it.
More than 10 bucks a round he will likely be sitting on it. 50 BMG guys generally reload too, so i would see if the brass is reloadable. I would also try to sell it in 25-50 rnd lots again lowering the price for the larger the lot.
That is of course if he wants to sell it.