Cinder is right- prices can vary from day to day on a server, and will vary a lot on different servers. For example, I could get at least 25 silver per stack of light leather on my Uldum characters, but on Sentinels, my characters have been getting about 10-12 silver.
The best way is to check the Auction House for prices. Simply go there, type the leather you want to check in the box, and click the Search button as if you wanted to buy some. Take note of the prices. Sell yours either at the baseline price, or perhaps a little lower than the others are up for if a lot of people have it up for the same amount. Basically, you're trying to make the price of your goods more appealing than everyone else's, so that someone will buy it!