At the moment the Attribute Token Trader only accepts 100 tokens and gives a +1 in the chosen attribute. If I understand the current NPC trading system correctly, there is no way to do a truly variable amount of tokens (i.e. you give any amount you have and get a corresponding increase), but it should be perfectly possible to add a trade option for 25 tokens that would give +0.25. In fact, you could just remove all the current trade options for 100 tokens and replace them with trade options for 25 tokens (it would be easy to convert any multiples of 25). This way players could readily convert their mission rewards if they so choose, since a lot of the missions don't give 100 tokens, but all of them give multiples of 25 tokens. As it is, you can't convert them even if you want to, becasue you need to have at least 100.