How does one go about establishing the value of a 30 stall horse stable, a 3/4 mile horse race track, a 2 acre pond and 25 acres of electric fencing?
@garyhowell - great question. If there are comps for any of those improvements in the area (properties with similar structures - which there may very well not be), you could try using those... but even that could be tricky because it still wouldn't be an apples-to-apples comparison.
If you can figure out what it could cost to rebuild those improvements from scratch, that's what appraisers do with the "cost approach" to valuing real estate (which is only one of a few different ways to value real estate).
Keep in mind, it could be a similar situation to a house with a pool in the back yard. Some buyers want pools (and thus, it adds to the home's perceived value) and others don't (and thus, it detracts from the house's value)... just because these outbuildings and structures are there doesn't automatically mean you need to offer more for the property. Depending on how well (or poorly) those improvements have been maintained, they could just as well decrease the property's value if they're not useful to you or the typical buyer in this market.
It reminds me of some landowners who will tout their property's "extra value" because it has a septic tank or a well... until you do more digging and realize the septic tank is 50+ years old and needs to be dug out and replaced and/or the well isn't functioning anymore... now you've actually got more expenses to cover if you buy the property.
As Seth said, assuming those improvements are still usable and functioning - like that electric fence - I would try to figure out the cost it would take to rebuild or reimplement those improvements.
The pond is interesting, because it's not usable land, and its usefulness might depend on the scarcity of water in the area (a pond in the desert is worth a whole lot more than a pond in a place like Minnesota that already has 10,000 other lakes around it). Might help to know if it's a man-made pond of a natural one.