As much as anybody is willing to pay for them.. there are only a few limited sets in game as well as real world items that are that specific brand. I don't recall how many they were limited to making in game but there were only so many for (M) and so many for (F).
+500 sounds like a reasonable price for them. Not as much as some of the rarer glasses in game but not as cheap as the cheap ones in game either.
There are only 18 sets of the GUNNARS that will drop, so I assume 9 sets of both male and female. As for price, no idea myself either (but I think people have been selling too low for the rarity of the item).