We really don't know if you are reaching a saturated list that's not hungry to sell, if there are too many others mailing the same list, if you're letter turns people off or indifferent, or if you don't touch them with enough frequency. .Is your letter targeted to a given problem or just a generic "I'll buy your house for cash (that I really don't have)". 

Perhaps the frequency could be cut down to save funds?  

Here is an amortization schedule download (you need to have Excel on your computer to open and run it) where you input your own numbers, including payment frequency, and that allows you to add extra payments, etc.