/shameless plug
I would stay away from Elance type sites unless cost is your only factor. 37signals job board will attract the exact opposite candidates as Elance (read: expensive/overpaid)
Ever think of hiring local?
Attributes to look for would be experience not only in creating the 'web 2.0 look' but also solid experience in eccomerce and application development. Too many designers only know how to make something look good, and fail to consider that you're doing business on the web. Avoid designers who rely on wiz-bang features, even a boring business like portfolio is better than one filled with tons of Flash and AJAX.
One of these days I'd like to actually put together a UI meetup. One day....
Hiring locally could be really good.. In general I think it's hard to find some one with an application portfolio though. Most are advertisement websites and it can be hard to gauge if their portfolio match your values.
Most UI jobs I see are either salary or long term contract. but I might be biased as that is what I look for.
Take Drupal for example - cheap AND good. Compare with our new Mega Corp CMS which had a 7 figure price tag and a work flow so atrocious it had to be scrapped before deployment was complete.
If you want good products then by picking something that is already good and only requires minor changes you have a pretty decent chance at a fair price (which might be high or low).
to a point. While design can often have a rang of price there can be a clear difference between paying some one a living wage and not. This number can vary international but it's good to know what is considered a professional wage abroad.
Also price can have a high correlation to experience. More experience is good if you care about the details but many rather stay cheap.
the design field is changing and there is a lot of price fluctuation and it can be hard as a company to gauge if a company is worth the price. But to say there is little correlation is a little misleading.
http://buytaert.net/drupal-usability-testing
It went through a usability test and surprise, surprise, the whole Drupal nomenclature is actually off-putting for the average Joe.
Usability is difficult. Very difficult. We'll see the fruits in Drupal 7/8. As a programmer/hacker/ia/ui guy -- I find all fascinating.
The general trend I think is UI is beginning to come of age.
Those places really aren't the place for $2000+ projects IMO.
It depends if its pure GUI or UI ... I think you should split up the project into 2... GUI is to get design and colour scheme. UI is to get the interactions you want and the flow...
We have used odesk, and we found that we can get GUI design done, but UI is really dependant on your target user market... This is difficult to outsource unless you really direct what you want done...
But I digress, the point of this comment is to try out 99designs.com. I've seen some really good designs on that site if you know exactly what you want.
We plan on experimenting with 99designs once we have our site up and running with UI we can come up with as developers and hope to get lucky!
In other words, you get what you pay for most of the time.
I saw, recently, a "screen" delivered to a company from a high priced design firm in SF that had ~800px wide text boxes for "Username" and "Password" on a login screen. They paid ALOT of money for that.
I think at the way low end you mostly get very inexperienced people who have little to no experience oftain coupled with little to no skill. They may commit to a price but can't deliver on quality. It doesn't seem worth it to waste the money here. Just have your developer do something.
At the mid range there are still a lot of unskilled people but there are now skilled people as well. Here are the people you need to find. Because they are going to be able to deliver something quality and worth paying for. But they are not doing to be able to match the skills of best designed sites.
The high price range there are still the same match of good and bad. the good are really good. Best in the business. The bad are from the mid range or below that are super confident or really audacious.
It's hard to pick out the good ones even though interview in my opinion. you really need a large list of references to get a feel for the "company".