I send out 176 postcards with each mailing and have received AT LEAST 1 client from each mailing. I create/cut my own cards (4/page) and have OfficeMax copy them on the bulk card stock I purchase at a local paper warehouse (marble print

).
176 cards - 4/page (44 pages)
Paper - $5.72 (.13/sheet @ $33/ream of 250)
Photocopy - $5.28 (.06/page 2-sided b&w - add $44 for color on 1 side)
Postage - $49.28
As you can see, the cost per mailing for black/white print is approx. $60. The cost if I add color to one side is still only $105. Not only is this a tax deduction, but even if I only get a 1-time 2-hour project from the mailing, it pays for itself.
To date, I have sent out 3 mailings and received 2 ongoing clients and 3 project clients. I think the key is making sure that your mailing grabs the potential clients attention...
1 of my project clients also referred me to another client who became ongoing.