The basic breakdown is this:
Cost of gun: $200 U.S.
Exchange at 1.20: add $40 Can.
Duty at border 30%: add $72 Can.
Freight from Asia: add $30 Can. per gun
Brokerage: add $10 Can. per gun
Customs inpection fees, license cost ($1250 Can.) are not included. This brings a $200 U.S. gun to a wopping $352 Can. cost to dealer. Now they have to add in fixed costs of running a business, like staff, rent, general overhead, phone and some even include the GST. On top of that, the license usually limits the holder to only 3 of any model of gun, so volume buying can be risky. It's very likely that the gun you see for $200 U.S. is being sold for $450 Canadian. In many cases that leaves less than $50 profit for the retailer.
In a volume sales environment, that's fine, but no retailer in Canada is capable of volume sales. There just isn't the market for it. I know that I wouldn't do it for so little profit.
BTW, I forgot to mention those occasional customs seizures that can cost you $5000 U.S. or more in lost product that you'd likely never get back. Every retailer has had to deal with that at one time or another. That's got to figure in there somewhere, too.