10 vangaurd marks for 20 mats =a total of 200 mats. If then you traded the 200 mays for crucible marks you would have 20. So in essence you're trading 100 vangaurd marks for 20 crucible. It would be extremely difficult for this to be exploited so I don't see how the balance would be messed up. There's still a mark cap of 100 a week and most items are more than 100 marks. And a side note, it would be cool to exchange mats for other items to. Like other mats, ascendant mats, or even motes or strange coins. (Obviously for a rate that doesn't break the balance)