Check out the Diablo 3 Auction Helper submitted by Tarran Rorem.
Description: The Diablo Auction Helper is designed to help you manage a set of items you plan to watch and to aid you in discovering profitable ways to change your diablo gold to real money.
Tracking items between the gold and real money auction houses has never been easier. You can record the cost of the item on the RMAH and gold auction house and it shows you the potential rates of return. Let's walk through a simple example.
For this example we will record the price for Immortal Kings' Will. For set and legendary items, due to the large quantity of them and the limited randomness of the stats, the worst item on the RMAH will be roughly equivalent to the worst item on the gold AH. Therefore we can sort by buyout and record the lowest for both.
The full description can be found here.