The appraised value from the county will give you a starting point, but that is normally calculated every 2 years, not every year and appraisers are now going back only 3-6 months.
If you do not need to get a loan, then you will need to find out the price per acre for similar properties in the area with similar improvements on them (ie houses, barns, wells, sheds, fencing, etc). Keep in mind that price is also based on usage - farm land for farm land, property to be developed for housing is different. It will also be different for a few acres then it will be for many acres. Try to find several similar properties then average them.