Ghost town sites are definitely worth it. Just recognize that you're going to have little to go on, unless you've got a map of the layout with some kind of landmark to use as a reference that is still there. Also, you'll probably want to run a series of straight-line patterns until you start picking up goodies and can map out the hotspots to focus on. As for it being a farmer's field, mostly the things that will suffer are some of the old buttons (some fertilizers will eat them up) and recent pennies. There will probably be a lot of iron you'll have to contend with, so be prepared for that. Chances are brass, copper, silver, and any alloy of those metals will have stood up fine. Gold isn't chemically touched by anything and looking for any old gold rings, jewelry, or coins would be enough reason for me to hunt a site like that. Good Luck!