gain from refinery minimal compared to gatherer bonus.
i use 15 gem rule for mines. if it costs more than 15 gems, it will take over 100 days per level to break even (unless you plan to play for the rest of your life and pass the account to future generations).
foundry is personal preference.
smelter kinda useless post-170 update when workers aren't used anymore for refines (i did 100 just because it's cheap and for short term benefit).
i use 15 gem rule for mines. if it costs more than 15 gems, it will take over 100 days per level to break even (unless you plan to play for the rest of your life and pass the account to future generations).
foundry is personal preference.
smelter kinda useless post-170 update when workers aren't used anymore for refines (i did 100 just because it's cheap and for short term benefit).