The response rates vary dependant upon the quality of your mailing list. As I noted in a previous post, for direct mail using USPS the response could be .5% to 1%. Lets say you mail 3000 pieces a month. 3000 x .55 (postage, envelope, printing)= $1650. A .5% response rate would = 15 new calls. If you close 90% of your calls, that would be 13 completed calls. Now you can visualize why even with this low response rate it is a good thing to do. Your cost to aquire that new customer is $137.50. That is a little high, but not unreasonable. If you can manage a $400 average invoice, your revenue would be $5200. That is a 3 times ROI. Again that is not great, but it's better than sitting on your butt waiting for the phone to ring.
Keep in mind that direct mail is a PART of a marketing plan, not the entire plan, but now you can estimate how many new calls you will be receiving that month. Need more calls? Send out 6000 letters.
If you want to find out a more accurate response rate in your area, call a direct mail marketer near you, they are in every community.