Hi Brian,
AFAIK, most decals are printed using the silkscreening process, which is basically laying down ink (or a series of inks) through a screen 'stencil' . This link shows the basics of the process: http://www.silkscreenbiz.com/silkscreen3.htm .It is a fairly expensive photographic process to set up the screens, mix the inks and produce the print, which is why most printing houses (decal or other) usually have minimums set in the hundreds of copies. You can print decals on ALPS printers, or ink jet, but those methods favour low number production runs (<20) due to the cost of consumables. Of course labour prices will be higher, but that can be reduced by doing the artwork yourself.
I don't mean for this to look like spam, but I make custom decals on an ALPS printer for other people, so could probably answer any questions you might have about making decals.
If you're interested in having a commercial run done, try contacting microscale.com to get an estimate. They used to have pricing info on their website, but you now need to e-mail them. Otherwise, there are a few places on the internet offering custom decals (myself among them) for various prices. For my basic prices, go to www.sparetimehobbies.com and follow the links to custom decals.
To everyone on this forum, I am open to all questions about decals, or any other topic.
Regards,
Pete