I had a great experience at 99designs, but it is much more expensive than what you’ve described. The cost is $139 to post a contest and offer the minimum prize.
However, I loved so many of the logos that were submitted to my contest. I picked the winner to be the logo for my photography site, and worked out a deal with another designer for a logo for my personal blog (it just didn’t work for my photo site).
the winning design
the other tweaked design
However, you have to put in some effort with rating and feedback for the designs. But I thought it was really awesome and I recommend it.
