It's not unusual for presidential candidates to spend heavily on direct mail -- as of January, all of the presidential campaigns has spent $18.6 million on such services. But in the waning weeks of his campaign -- even as insiders well calling on him to drop out -- former Sen. Rick Santorum (R-PA) spent almost $1.9 of his supporters' money trying to generate new supporters with direct mail, according to FEC documents filed last week.

Santorum's campaign paid a dozen companies in March 2012 over $1.54 million for everything from direct mail campaign design and fulfillment, caging (a way to pull potential names to whom to send mail) and list rentals -- and owed six of those companies more than $342,000 for expenses incurred in March. For a campaign that was bragging that its viral sweater vests netted 3,000 takers at $100 a pop and only took in $5 million in donations in the same period, $2 million is a hefty chunk of change just to try to reach new donors.

And direct mail, while perhaps more successful than unsolicited emails, doesn't exactly have a high success rate or quick turn-around. Yori Wurmser, the Director of Marketing and Media Insights at the Direct Mail Association, told Raw Story that their research shows that each regular-sized envelope from an "house list" of current donors and people with an expressed interest nets on average response rate of 3.42 percent; from prospect lists, which includes rented lists of names of people deemed to have similar interests, the rate is 1.38 percent. Wurmser added that, for such campaigns, "Lead times can be several weeks, in any case." While postcard solicitations score higher response rates from potential targets -- 3.99 percent from house lists and 1.72 percent for prospect lists -- that's still a lot of wasted paper.

Wurmser added that the cost per lead generated averages $96.01 for a regular-sized envelope and $75.32 for a postcard. "You have to have a justifiable return on investment for that, based on the lifetime value of the lead," he said. By way of example, he pointed to credit card solicitations: "A credit card company is willing to have a really low response rates, .4, .5 percent, because the lifetime investment can generate thousands of dollars per customer."

There's no public data to show how much Santorum's successful direct mail solicitees donated -- though, by Wurmer's numbers, each person who did donate would have to cough up $100 for the campaign to net $4, if the campaign's expenses were average. (Obviously, the sweater vests, which were given to each $100 donor, would drive the cost of each lead up.) Plus, to make the return on investment worth $2 million in direct mail costs over time, those people who did respond with donations would effectively have to keep donating -- but Santorum dropped out April 10, 2012, just ten days after the period covered by his most recent filing, and long after it was clear he wouldn't have months left in his campaign.

But direct mailing lists can do more than generate donations for candidates: as former House Speaker Newt Gingrich's (R-GA) recent move to rent out his 338,000-person donor and activist lists shows, the lists themselves can prove quite valuable on the open market. And, of course, lists can function as a way to keep in touch with or solicit supporters for a candidate's new projects, be they campaign-related, campaign debt-related (as Hillary Clinton supporters likely already know) or none of the above.

By way of comparison, while Santorum's campaign was shelling out nearly $2 million for direct mail in March, they were also paying Republican ad firm Brabender Cox just over $2.4 for ad work (including the "Rombo" ad with the poo-cannon and a series of introductory ads) and yet more direct mail not included in the $2 million total. By comparison, the campaign only spent about $631,000 on charter flights to ferry the candidate and his staff all over the country to generate the delegates he needed to have won in order to credibly stay in the race.