Discovered this by accident, not sure on which items it's worked, but I have tried it on Precise Cardinal Ruby and it worked, but on Ornate Topaz, it didn't maybe its quality of gem? I don't know, but anyways, here it is...
When you put up an auction, for me i did 1 item per stack, and clicked the max stacks button, filled out all the boxes for my auction (BO, Starting Price, length.) And I went to check average prices so I could sell the item quick, and when I went back to the auction tab, the cost of the auction setup split to be as if I were only putting one auction up. When I put the auction up, it only made me pay as much as if I were putting one auction up, (I looked at gold in my bags as I clicked post.)
Now this may or may not be useful to you, and depending on which items you are using. It might be helpful if you are doing a bunch of auctions at once, or if you like to keep all the money you can, or whatever other reason you can think of. Whatever the reason, I hope it helps, will do more testing once I get more quantities of gems. Enjoy.
(Will Uploads Pics Later, when I get them)