I'm crowd-sourcing my book's cover using 99Designs. You should click the link to see how clever it is. I pay them £300 then a bunch of designers take the brief for my book and go off and quickly come up with a draft design. I then chose a few finalists in the "contest" and those folk polish their draft. I chose a winner.
It works for me because I get a whole lot of different ideas for a fixed price - and If I want to attract better, or more, submissions then I pay more - i.e. set a bigger prize - so that it's worth the better designers' time.
It works for the designers because, although they don't always win, their drafts don't take much time and if they're good enough they'll win some of them.
It reminds me of how Zara does product development: makes loads of stuff, try to sell it, if it sells then make more.