I know there's not a cut-and-dried answer to questions like these, but there might be some guidelines I should be thinking of and it may help others as well. The case is as following:
I run a wiki community with a few thousand articles, and I may want to have it re-designed in the near future. Currently it looks pretty okay but there's a lot to improve especially when it comes to usability and a consistent style. Pre-fab designs are almost never what I'm looking for, especially because the website is so content-driven and focus must lie on good readability of the articles without losing the thematics of the subject (a rock band). Imagery and a certain dark style are very important with this band, so a good balance between readability and style will almost certainly have to be a custom job by someone who knows about these things.
The current implementation counts two basic pages (main page and article pages) and some variants like article edit pages, user settings etc which are mainly article pages with some extra elements. Apart from this the article contents can have so many kinds of elements (infoboxes, templates for different kinds of related content, etc) these will have to be designed too.
What should I look for when searching for a designer? What are good price indications, how can I tell if the designer knows about usability and can deliver what I want? Can I ask him to give me a first draft (for a small payment of course) to see if he fits the bill?