All land belongs to someone. Or some entity. Without permits, environmental surveys, etc, you would probably end up in more trouble than it's worth.
Now you could BUY an old townsite but in many cases the current owners have warped visions of what their land is worth.
For example, the current owner of the Swastika Mine is trying to sell her 40 patented acres at the same price she paid for the original 80. I was told the going rate for land like that is around $2000 an acre, although in this economy you could probably get more for less, especially if you have cash.
What I said about all land belonging to somone:
Tip Top - private property, patented mining claim
Gillette - same thing
Swastika, Peck, DeSoto, Oro Belle: all the same thing. Just because they are deserted pieces of land doesn't mean someone doesn't own them. The DeSoto and Peck Mines are HUGE holdings of land which indeed have owners. Why anyone would want to own something like that, pay taxes on it every year, yet DO nothing with it defies description.
So whatever locale you are looking at most likely is the same. In a National Forest, it's easy to tell by looking at the map to see if the land is private or public.
My favorite story is about the Orizaba Mine, located in Moores Gulch out by Table Mesa. Used to be an easy drive back in the 1980's. Ran into a huge dozer out there in the early '90's working on the road. Then as the years went by, the road deteriorated until it was a rough trip to get there. Three years ago I went there with some people in the Bronco Club - I was the lead driver - and the further in on Moores Gulch Road we went, the better the road got.
That, of course, made no sense until the got to Orizaba and found a full sized Santa Fe style house, looking like something from Arrowhead, sitting out there in the middle of nowhere.
Since there was a gate (not locked!), we opted to turn around and go back.
Last year I was working on a trail in the PNF and all of a sudden this guy shows up out of nowhere - he had tracked me down by the sound of my chain saw. Turns out he bought Orizaba from the State; then he also had to buy a right of way on Moore Gulch road as well, and built his totally off-the-grid house (complete with internet, phone, AC, indoor plumbing !!! the whole package!).
I didn't pry as to what it cost, but he must have been well-off to pull this off. BTW his name is Keith so if you ever visit, say Hi. He seems pretty decent....
Anyway, I would be very leery of doing what you are proposing.
What really peeves me is how people are steadily destroying what ghost towns are left. Table Mesa is a battle field; Gillette and Tip Top have been pretty much destroyed. Even Oro Belle has been vandalized by all the flatlanders who seem to have zero respect for history.
End of rant