1 - Spend a bit of time searching for similar items and see what they are going for.
2 - Set a buy-out price that is about 25% cheaper then what people are asking.

Most people paid for the game to play it so anything you get from the RMAH is all profit. Even if you bought the game for the singular purpose of making a few bucks, getting the purchase price back should not be a problem if you undercut most other people. I've been doing this with the gold AH and making a pretty good amount of gold. I also have been checking auctions for ones with low buy-out prices and snapping up really good items cheap, then turning around and putting them out for auction again with a slightly higher price... Not as much profit but still a steady stream of income. I've gone from about 40k gold to over 300k with almost no play time, just turning stuff around at auction.

Bottom line is you are going to have to do the searches yourself to see what your items are going for to get that baseline price.