Check the link for info on dungeons. Dungeons can be bought for bananas and so far 5 shop basement dungeons have been bought for costs between 2 million (20,000 ped TT value of bananas) and 2.5 million (25,000 ped TT value of bananas). As for final cost that can differ with what markup was paid for the bananas. The next shop basement cost is 2.75 million bananas (27,500 TT value). One camp crunk cloth crate was also purchased for 2 million bananas with the next one costing 3 million bananas.
As far as profit/ROI goes, would have to seek that information from the 4 current owners of the dungeons.
Current costs to obtain your own, depending on dungeon are currently from 27,500 ped all the way up to the most expensive at 500,000 ped plus banana markup.
http://rocktropia.com/bp-keys.html