This is pretty cheezy. 

For example:

At gamestart a General can buy level 1 monks for 1000 gold, summon the monks, and then sell the idol back for 800.  Then he has 800 gold to spend on combat pots or armor.  In a starting 1 v 1 flag duel the general who does this will always have an advantage over his opponent, and he will have paid 200 gold for it. 

Rinse and repeat.  You end up with an army 80% more powerful than you paid for.  Good thing no one is using minions too much right now.  Hopefully that doesn't change 

Let's not even get into what organized feeders could do with this system. 

To fix - Idol-summoned minions should only stick around as long as you have the idol.  Sell the idol and they die. 

*Thanks to Micah for testing this.