Without doing hard calculations, each star is adds roughly 12%. If you trade in 8 stars (the max) your reward will be a little bit less than double. I look at all my levequests before trading anything in. Look for your best reward first, then trade as many 2 star cards as you can (4 max.) If there are not 4 cards with 2 stars, I will save the other cards for a different leve where the card is worth two. One completed levequest card can give different star values for different tradeins. When you try to trade one it, it might give you 0 stars for one leve, and 2 stars for another one. General rule; don't trade 0 star ones, always trade 2 star ones on high reward leves, and trade 1 star ones if you have no other option.
You should also be aware that you will forfeit your trade in reward if you pick the same levequest as one you already are holding. For example, If you just completed 'Just one of the Dodos,' and when it becomes avalible again you pick it first, it trades automaticallyI'm pretty sure you lose the reward. This is important, because if you traded on a different levequest first (like for 2000 guildmarks)you will get a better reward (roughly an extra 120 guildmarks per star - thats potentialy 250 guildmarks you throw out the window by having the levequest automatically trade.) If you didn't pay attention at all and did this with all levequests, you lose all the bonuses of trading them in.
Edited, Jan 27th 2011 9:52pm by WolfOak