'Tis the season I guess. Actually a helmet just came into the shop today for some paint and a light chip repair.
Prices could vary a bit depending on several things. First and foremost every sticker (if any) must be removed from the helmet first to prepare it for prep. This doesn't actually make it more expensive though, just part of the process really. Its best that the painter remove these to keep from scratching or chipping the helmet in any way, and also so he knows exactly what that sticky residue is that is left behind.
Helmets are tedious work, and some even say a labor of love. Personally I like painting helmets.
Graphics will be somewhat of a consideration in cost. Naturally elaborate airbrushed scenes are going to take more time and the painter will have to factor this into his price. A logo, or even simple graphic like flames, scallops, emblems or something of such is relatively easy to create and shouldn't take much time at all.
Of course most painters would have to see the helmet and understand exactly what you want before being able to give an accurate quote, (myself included) but helmet painting typically averages $110-$500 depending.
By the sounds of it, you want something simple so you can expect yours to be somewhere in the $100-$150 dollar range.
This doesn't include shipping however, you'll have to contact UPS for that. Also if you're planning to ship your helmet, it helps to have the original box it came in. If this isn't available, you might have luck at anyplace, (including Walmarts, Targets, etc.) that sells helmets and ask them if they have a box that a display helmet came in. Sometimes they'll actually give you one. The advantage there is if they leave the padding inside the box your helmet won't bump around. There are other methods of course but that is a simple consideration.
Hope this helps.
BadgerOctane