If I trade my droid in how much will it cost me to get the incredable?
First, I'm not sure why you'd want to do that. Both phones have their advantages, but those are quite minor. Except for a few uncommon scenarios, there's no compelling reason to switch from one to the other. The two phones are far more alike than they are different, from a functional perspective. If anything, the huge user base of the :motdroidvert: gives it a slight edge, IMO. The second biggest difference is the Droid's longer battery life.
Anyway, if you are sure you want to switch from the Droid to the Droid Incredible then your best bet is likely to outright sell your Droid and buy a Droid Incredible without a contract. The Incredible will run close to $600, and a used Droid will fetch a little over half that much on a site like ebay.