If it for an untargeted market, i.e. a list of everybody.. Expect .001 to buy, that is 1 out of a 1,000 so if your cost is $500 bucks to send 1000 out, then you better be making more than $500 a sale..
Now if it is a niche that you are sending to, such as pet supplies wholesale and you are sending to independent pet stores expect a better response..
I have sent out a million postcards in the last 10 years.. I was in a niche market... Make sure you are too..
My opinion is if you are going to make $15 - $40 bucks per deal.. Postcards do not work..