When I send out $1000 in post cards and get two "come list me" calls that is what I am paying, right? 

Get some postcard/business card combination and hand write a note to the owner asking if there is an opportunity to buy his property located at (address).

Self Advertising: Business Cards, Social Media Posts, Mailing post card styled ad to renters/people looking to buy, and mailing hand written notes to past clients.Business Cards: Hand out after conversations, put out in local stores/restaurants (after receiving permission) and put in all flyers/ads/notes mailed.Social Media Posts: Put out helpful information and use colorful images/texts to get peoples attention.Mailing Post Card Style Ads: Will advertise that you're willing to help new clients.

When I first started I was marketing directly to homeowners with 'We Buy Houses' type marketing (bandit signs, letters to owners of vacant houses, postcards to absentee owners [tax bill goes to different address], doorhangers, flyers, etc.).During 2009 and 2010, I was buying mostly REO's.

Or, are postcards more effective?

I have bandit signs that stay up for awhile but I also get calls weeks or in some cases months later from leads that held on to my post card or letter.

Our 2 best sources for buyers - postcards to absentee buyers in the same zip code for the last 6 months and 20-25 signs in the area...good luck

Yellow letters vs post cards, short copy vs long copy, glossy vs plain text.

Maybe I will look into the postcard for absentte owners...

If your farm is very large, which it appears to be, I suggest you use post cards.