I did this with my postcards and that's about the return I got too.
I started a thread about this last year that has lots of information-I've done the postcards to T65s with just a call back number and averaged 1 sale per 400-500 cards that cost just under 60 cents so that's about $270 in client acquisition cost. I can't say why I haven't done more of it, maybe because there is a limited number you can send each month in a specific area and you wind up with one sale a week or so.