On my first couple deals, I got properties under contract for about 40 or 50 percent market. Luckily they had title problems and I couldn't close. Then i realized there were more fish in the ocean. The number one thing that protects a land deal IMO is getting it dirt cheap (another is cheap holding costs-taxes). So basically, if you get it cheap enough that you don't care, you can alway sell it and profit.
I have also found that if you send enough mail, someone will accept, or at least write back and negotiate their price. I also have found in FL for building lots I need to pay about 30-35% market value, maybe because there is more competition. Other rural markets I can buy for 15-20% easily.