It can be very difficult to get a reliable appraisal on undeveloped land like the acreage behind your house. In most places, the value really depends on how many buildable lots you can get out of it. Usually, the number of buildable lots is driven by many factors such as the terrain, water/sewer vs well/septic, required setbacks, storm water management requirements, etc. Developers are generally the best equipped to come up with a reasonable guestimate and only after investing some engineering hours into the analysis. This generally exceeds what a typical residential realtor can handle.
The bottom line is usually how many buildable lots end up incorporated into an approved development plan which can take months or years to acquire.
On the other hand, if the land is relatively flat and has an easily dividable shape, zoning requires say 1 acre lots, and everybody is on well/septic, then maybe you can estimate the number of buildable lots with some certainty based on the land area alone.
My point is, to estimate the value, you need to know how many houses can be built on it. And that is sometimes difficult to guestimate without some engineering work up front.
Before selling, I would try to contact a few smaller residential developers (assuming zoning is residential) and see if they are interested. Solicit offers from them. It's quite possible, the realtor is considering doing the development or possible they want to flip it to a developer at a profit. You can just as easily sell directly to a developer.
When my parents passed away with a small 10 acre farm in an area with massive residential development occurring, we sold directly to a developer that was already doing development next door. It allowed them to expand their development plan to include our 10 acres. So it had more value to them than to others. You want to find the developer with the biggest incentive to buy your property.
Sorry for the long post...