as stated in the original post, until i figure base costs, as well as time and risk factors, i cant rly say.
How about a Pigman farm?
I'd definitely pay for one of those
A good investment
50k maybe at the most
as for the pigman farm, i cant say, as i tend to avoid the nether. prices would ultimately depend on efficiency. a 1 or 2 tier spawner would be cheaper then one that stretches from bedrock to bedrock. (tho id prob put the max at 5 tiers or so)
How much do you think one would be to construct? You could calculate costs based on size, with a minimum and a maximum size required for purchase (to make it worth it or something).
If people find their own land, does that count for more?
with enderman farms the basic construction can also be only 2 or 3 lvls, but most ppl make them from y=0-120 or so. the basic questions would be height and width. ill need to consider what is sensible b4 i can rly say anything. tho if the person wants to make the original platform, the price would drop by a slight amount, as getting 250-500 blocks out can be a real pain in the end