Mystic Pond
The mystic pond, a Rampart structure, produces 1-4 of a random Resource except gold on day 1 every week.
Requires: Village Hall
Cost: 2.000 Gold, 2 Wood, 2 Ore, 2 Crystal, 2 Gems, 2 Mercury and 2 Sulfur.
Value Analysis
If you consider basic/magical resources to be valued at 125g/250g (9 marketplaces) then the gold value of the Mystic Pond is 4500g and the average weekly value of the randomly generated resources is 520.83g with a payback time of 8.64 weeks to recoup the value of the initial investment before you start turning a 'profit'.
This can be compared to resource silos, which have a payback time of 3.21 weeks (22.5 days) if you also use the 9 marketplaces values for resources.
If you redo the calculations assuming you are buying resources out of one marketplace (2500g/5000g per resource) then the payback time is 'only' 4.99 weeks.
The Mystic Pond is only useful for two things: 1. It counts as an additional building so it will increase the damage output of your arrow towers in a siege 2. The map is incredibly resource poor with close to no mines and/or very limited loot-able resources
Note that even when games last 3+ months the Mystic Pond may still be poor value since the value of the investment must be front-loaded (you must invest in it very early on) and those resources would almost certainly be better spent on troops or other buildings. Considering all these things the Mystic Pond is nearly always a poor investment.