yellow letters or post card

you may want to fill out some info on your profile; that's the first place other investors on this site look to see who is the investor that is asking the question and where is he from?

You have to try both the yellow letters, and the postcards to find out what works best in your area.
You also have to do several mailings, not just one. It is not cheap to do either. I would estimate about about 39cents/letter plus cost of postage. Postcards at Vistaprint start at around $10 for 50 for the basic, plus add the postage as well.

Someone recommend me to check out ListSource dot com for doing yellow letters. Like Valuni mentioned above, its not cheap but you will get a good response rate if you do it a couple of times.

Listsource is a good source to get your absentee owner lists from.
Criteria:
-owned property for at least 4 years
-at and below the median home price for your area
-2 to 4 bedrooms
-900+ sq ft

If you are wholesaling you might want to up the time owned to 7 years so there is some equity or
select 50%+ equity when you are targeting your list.

You have to mail more than once. 6 times at least to one list before you recycle it. I would suggest sending out 3 letters at least (once a month) and then you can mail postcards to follow up/build on the letters.
Cost per lead for a postcard is on average around $100 while cost per call for a yellow letter is $20.