$35 for general custom designs from someone like Extreme Pixels to several hundred for a purely custom job. Mostly depends on what you'd want and how graphic intesive it is. If your design is simply customizing the style sheet then probably not a whole lot but if you require heavy graphic work that makes it necessary to start changing all the templates that could get pricey.
Well, a complete redesign of a forum. About 15 high quality designs, custom scripting to allow multiple forums on a website to be browsed under one login ID, custom scripting to allow users the opportunity to refer other people to the forums (referrals would be tracked by the script and added to the users profile), custom scripting to allow moderators the ability to add awards to user profiles, custom scripting that would show the latest posts on the index of the website. What would the price range for a project like this be?
You post your work on freelance websites and start a bidding war among freelance web designers. You can then go with the one who offers more for the buck.