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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It should be based on the rarity of the weapon.