Since you are not converting any, the economics part of the equation is moot (at least until you either solve the problem, run out of money or stop).The solution is to either increase your frequency of touches and narrow your message or choose a list that isn't so picked over.

Post office will run it through their NCOA system Clean it up i.Sort by last name1.some people won’t have last namesa.remove trusts, corps, llcs, inc, etcb.Use “family of <last name>” Send list to yellow letters What I want to send Frequency Let them do their magic What do I send Small 1st class Postcard i.Needs a call to action1.Urgent Notice2.includes name3.phone number4.owners city5.multicolored – red, green6.back of postcarda.Attention, call to actionii.1st monthiii.include cost of list in budgetiv.will get 6% returned to us undeliverable1.people have moved2.haven’t updated post office3.only get back if 1st class stampa.1st class-treated with priortyb.2-3 days priorityc.will send back to you if bad address4.never use a pitney bowes machinev.we want to get mail back of bad addresses1.try to find owner, PI, skip trace2.find about any liens, taxes, etc for abandons3.a really good resource for prospectsvi.MQ doesn’t remove them from the list…you may want to1.on a tight budget remove them2.if not, you will get new list in 6 mosvii.Takes 5 business days to produce and mail Yellow letter i.Goes out 30 days after 1st pieceii.Looks to be handwritteniii.Red inkiv.Invitation envelopev.Use different phone number than 1st piece1.want consumer to believe 2 different people are marketing to them Larger postcard i.Property Address Noticeii.Phone number in red ink with arrowiii.Final Noticeiv.There is a better way to sell your homev.It is twice size of the small postcardvi.Uses a standard stamp1.won’t get sent back if undeliverable2.will take longer to get to owner3.bigger than a standard envelope Zip letter i.Message same as large postcardii.Has to be torn openiii.Looks like jury duty, govt noticeiv.Very official lookingv.Tear away that they can return on bottomvi.Gives the prospect 4 ways to contact you1.older people like it Professional letter i.Use logoii.Handwritten signature in blueiii.Similar messageiv.Big points in boldv.In a #10 envelope Greeting card i.Put a business card inside itii.Match the month your in1.Christmas, easter, mothers day, 4th july, thxgiving, etc2.all about nurturing

This is only because of the frequency of the smudges. 

Also, I think I read somewhere that you need to send mailers at least 5 times to really get mind space, so frequency is very crucial.

The frequency really depends on the class of property and tenant but every property should be inspected at least once a year.

If they don't respond at any point, maybe give them a little time and repeat with a little less frequency.  

Here's a quick tip to avoid a potentially costly disaster with your letter campaigns:Whatever strategy you go with, whatever area you target, whatever frequency you decide to send your letter campaign, KEEP GOING and DO NOT STOP.