Would love to be able to choose what elemental type I want on my weapons, so I thought why not be able to buy upgrades from zur? but you can only upgrade exotics otherwise they would lose their 'exotic' status if all legendarys had elemental damage.
I'd say they would have to cost around 50 strange coins as these types of upgrades sufficiently increases the weapons potential damage.
[spoiler][url=http://www.bungie.net/en/forum/post/74973306/0/0][/url][/spoiler]
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I don't hate this idea, but they won't do this since the main benefit to raid primary/special/heavy weapons is that they have elements associated with them to help with shields or burn modifiers. If you could choose yourself you'd just end up with people who buy the same weapon 3 times with marks and set one element to each, eliminating the point of having true multiple weapon sets.