Some designers charge 100 while some charge upwords of 500+ All depends on what you want and how soon you want it. Then again, you can always purchase a premade style for 15-40 bucks, but are those really custom? I don't think so because hundreds of sites will have the same exact skin as your site would have. You should definetly have one designed soely for your site. It may cost a bit more, but its better than those ones that are sold out of a box. I normally charge about 150.00-175.00 for custom skins and I've designed sites like:
http://www.thehostingbox.com/forums/ ,
http://www.muscle-mustangs.com/forums/ ,
http://britneyspearsworldwide.com/, etc etc.