I often forget until GenCon rolls around but if you want the productions costs of your prints to be really low you need to plan for the images being 11x17 or 8.5x11. You have a lot more options as far as companies when it comes to looking for those size prints and most of them offer big price breaks for ordering as few as five. It's not too hard to get each of your prints for fifty cents to a dollar. If you really go nuts and order a lot of prints, some places will do offset printing which is supposed to be higher quality. I ordered some prints last year that were offset printed and I honestly couldn't tell the difference.
If you've failed to design for those sizes you'll end up using a company that prints for photographers and prices are much much higher. An 8x10 might cost 1.30 to 2.50 and usually these companies don't give price breaks until 10 prints and even then it's not competitive with the 8.5x11 prices.
There aren't really any cheap options for the larger sizes such as 16x20 but you'll have to go with one of the photo places.
There's another strange thing to consider that had never come up until last year but you might want to ask the company if the prints can be signed. Or ask the company to send you samples and they will and then you can test them. Last year my prints were impossible to sign. All inks just beaded up on the surface. People would ask me to sign their prints and I felt bad because they were left with a pointilism version of my signature.
Thought to post this because I didn't order any prints of my Mage piece because I made it a really strange format. Really silly considering it was a private commission and I could have made them any dimensions.
If you're going to GenCon don't forget to check out my GenCon recap on the blog as it might have some useful information for you.
Right now I'm working on two traditional media pieces and I should be allowed to show them in August. I'm excited and hopeful about them. Thanks to Lynda.com I'll also be learning Zbrush better soon. The website isn't as close to being finished as I thought.