Direct mail can be vary costly and I think the list must be targeted very specifically. The best response rate I have seen from direct mail has been about 2%. Usually when I do mailings I do them in smaller blocks of 500 - 1000. This way I can track the types of calls I'm getting and I can change the message, medium, or market as needed. For example if you don't get an acceptable response rate you can change the actual message on the piece you're sending out. You can change the medium: postcard, yellow letter, etc. Or you can change who you are sending them to: the types of houses, geographic location, length of ownership, tax value, etc. Also, where are you getting the mailing list from? I bought a 5000 name list from an internet company for $400 and it was useless. I now use a virtual assistant to build lists for me via odesk for a fraction of that cost (I'm more than happy t refer her). Believe it or not there are many variables to consider.
I have found bandit signs to be the most cost effective for getting immediate seller leads. Personally I like to use multiple forms of marketing to get the most leads coming in. I don't like putting my eggs in one basket with regard to marketing.
$10,000 is a lot to spend on one marketing medium that has yet to prove itself effective. A few questions I would ask myself is: What is the average profit for a wholesale deal in your area? Are you looking for cash "MAO", or owner financing deals? Do you post ads to craiglist regularly?