This information is over 4 years old and should only be used as a reference, NOT as a pricing sheet.
I had some done in 2012 and this is sort of the way it went.
Since I had the artwork, it was a matter of getting Adrian Gideon to set-up the sheets, which cost $15 per sheet. If he had to tweak the artwork at all, it would have cost another $10 per sheet. His preference at the time was for any art submitted to be in .eps (Encapsulated Postscript Vector graphics (Adobe Illustrator)) file format. Also because I already had the artwork we never talked about custom art prices, which would of course have been extra.
Note: the price to set the artwork up is just that. It does not include any actual decals. But once you pay for set-up (which I'm pretty sure is a one time cost), you pay the same price for your custom decals as you do for the ones they have listed online. I also want to say that Piranha (who makes the actual decal) saves your decal file in case you want more later on, making re-orders easier.
Things change, so I have no idea whether any of this is still the case or not, but it is how it went in 2012. I do know that since then decals took a small increase in price, so Adrian Gideon's prices may have too... Bottom line though, they were great to work with and the decals are certainly worth it.
Caz