Aug 19th 2009 6:11PM I don't see why it can't just be a normal business transaction. If the vendor price of the drop is (v) and the shard price is (s) just pick a price (p) somewhere in the middle. If you want a shard, you pay the enchanter and you're getting a low priced shard and they get a nice amount of gold. If you want the money you sell the drop to the enchanter and they get a good price on the shard.
This way everyone wins demonstrably. To wit: Let's say the drop (v) is worth 5g at the vendor and the shard (s) is worth 25g at market price. Furthermore, assume the negotiated price (p) is 10g.
If you win the drop and want a shard, you pay the enchanter 10g (p). You now gained 10g (s-v-p) over vendoring the item. The enchanter gets 10g for her work.
If you win the drop and want the gold, you sell it for 15g (p+v) to the enchanter. You now have 10g more than if you vendored the item (s-p-v), and the enchanter gets a discounted shard (In fact they gain 10g, since they could sell the shard for 25g).
In both cases the value of the loot is increased from 5g to 20g (s-v, since the item is lost). You'll also notice, that both parties split that gain evenly (increase of 10g).