i've seen motorcycle helmets that look like a skull or a devils head. is that just a plain helmet thats been air brushed or are they bought that way? if it is air brushed what kind of cost am l looking at to have it done? and does the paint hold up?

You can buy helmets that have designs on them already, or u can have it done any were they do custom airbrushing. Prices will very on the place that would do it. Around here they have places that airbrush shirts & some of them will do a helmet to, u probably have to prep it before the paint it though.
Here r a few sites. Just do a search for helmets & u will find a lot of sites.
helmetguy.com
sstmotorsports.com
buymotorcyclehelmets.com
skullscreamers.com <--- really cool helmets