I don't think the idea of just randomly sprinkling chests around the overworld will work because players dig a lot. There are instances where players have found event chests before the event was announced, for instance.

Changing the amount displayed is definitely a good way to somewhat determine the difficulty, but what I meant was that all it takes is one landmark to give it away. So even the hardest difficulty with a large reward could be easier than the easy difficulty if it had say... a corner of the MoD Carnival tent in it.

The idea of a medium between randomly generated maps and being reviewed/approved would probably be the best bet. Still would be somewhat involving, as I'd have to sit and click through and approve/deny until a heft sum is generated. Also I'd have to script up the system for it, but it'd probably be the route I take.