John: Probably Inkscapeforum.com is a good place to start. Anyway, I'd suggest you to think twice before proposing such contest. You're offering a very small amount of money as a price (far below the street price of a logo, even for my country where our currency is 1/4 of a dollar) and these kind of contests are perceived generally as a cheap strategy to get several logos offering almost nothing. It may damage your reputation among the designers and you'll only get design proposals from hobbists with a poor or non-existing knowledge about design. Contests are generally considered as spec-work, and serious designers really hate that.

If you don't have money, maybe the best thing is to announce that you need a logo, that you have that money and no more and let the people interested decide if they want to design something for that amount. You could ask for a portfolio of the interested people and choose one, and from that point continue working as a regular contract work.