There's way too many variables to take into consideration for me to give you a price here.
Server population, faction distribution, and class distribution all help determine what goes for how much.
On my server (Trollbane-Alliance), Light Leather goes for about 20s/stack, and Medium Leather goes for about 29s/stack.
That is not to say they will go for the same on your server. My server's low pop, and weighted about 65% Alliance. My old server, Elune-Alliance, Light Leather would go for 40s/stack, while Medium would go for s35/stack. (No, that's not a typo. Elune's economy was totally screwed when I left.)
I'd really have to recommend getting the
Auctioneer add-on, and using that. It's very helpful,and has made me many small fortunes over the past year and a half.