Costs of marketing via Direct mail campaign

[Lana & Tyrell K.]

So as I am learning about shotgun marketing I am hearing that the process in general could be quite expensive yet no one has explained specifically what parts of the shotgun style of marketing actually IS expensive.

Besides the cost of stamps, post cards and envelopes, (and I'm told I also need to purchase a $150 permit from the US postal service in order to send bulk mail) what other expenses do I need to take into consideration?

I also heard that when sending out postcards to my different lists(probate, tax delinquent, absentee owner) that I need to keep in mind the law of averages (ex: 1000-2000 post cards mailed =100-50 returned phone calls= about 3 deals under contract) Has anyone else heard of this formula or know it to be true?

-Tyrell