At max lv, I'm currently getting +6 every 8 hours, minus the Prestige bonus that would be +5 every 8 hours. That would be 15 gems every day, it is the same from completing the daily quest. Keep that up and you can summon x10 3-5* heroes every month just by having Gem Cavern alone! I didn't even include the expedition yet, 100k gold for 10 gems with a wait time of 12 hours.What is the daily rate of the gem cavern if I may ask? I may be the strange one to not want the gem cavern. I was thinking about these setup.I just want to stress that Watermill is extremely important for everyday progression. A max Lv. Watermill gives 72 energy per day (Rate is higher depending on Prestige Lv.) plus another 30 from expedition. It can be extremely helpful for farming event currencies to trade in the event bazaars. I would strongly advise everyone to get it whenever you are able.
Small : Wishing Well, Watermill, Strike Tower, Shield Tower, Mystic Tower
Medium : Armorsmith, Salvage Yard, Market Place
Large : Weapon Forge, Fusion Altar
You're essentially getting premium currency for free, who wouldn't want that? You have to sacrifice either the Weapon Forge or Fusion Altar, you will hardly use them, the benefits of having Gem Cavern very far outweighs those 2 buildings, think about it.
Also, if you're planning to keep the Marketplace, you have to be sure you can keep up with their rates. Whatever they are selling right now is just plain daylight robbery, you might want to consider getting an Academy to help with your hero progressions.