I'm at third cave,Tin cave level 6,almost level 7.My upgrades for this specific one is level 3 Copper and level 6 Tin.For drops,I only have level 3 Stone and level 2 Tin.100 energy cost me 227MCC and get me only about 150-200 MCC so yeah,I have to wait each time a while before I can buy another one especially when broke.What do you think I have to upgrade to fix it?
Have you tried any of the upgrades from the Mine Resources tab. The ones specific to that cave? They multiply with any permanent upgrades you have.