I've bought quite a few pieces of farm ground through realtors, direct from owners, etc. It's really not all that difficult to get a feel for what the ground is worth. If you have a relationship with a local bank, they most likely have an AG lender who could give you an idea what the ground is worth, based on comparable sales in the area. An AG appraiser working directly for you is another good resource to get this information. Be aware however, that loose lips sink ships and the more people that know about the place being for sale, the higher chance someone will come along and snatch it up ahead of you. A lot of farm ground around here changes hands without for sale signs ever going up.
In the end, it boils down to how little the seller will take and how much you are willing to pay for it. I always start with a pretty low offer, maybe 75% of what they are asking, and then just go up from there till you hit on a price both are comfortable with. If you need to finance it, make sure you have that lined up in advance or your offer isn't good for much. Include significant earnest money as part of your offer (maybe $10,000) so they see how serious you are. Don't complicate the deal with a bunch of terms and conditions and things the seller has to do, just accept it the ground and buildings as is.
Probably the most important thing is to do a title search so that you are assured their are no other liens and judgement against the property. Most title companies that handle these deals do this as standard part of the deal. If there are currently unharvested crops on the place yet to be paid for, you will need to decide who gets paid for the harvest. It's not unusual to let the seller go ahead and receive the crop payment since it was his ground when they were planted. Real estate taxes are generally prorated and settled at closing, then tax bills going forward are on the new owner.
How many acres? What type of ground? What state? Any wooded ground? Any tillable or hay ground? Is it an area ripe for housing or commercial development?
A little of this info would go a long way towards some more detailed advice.