Of all units monks give you the most bang for your buck, in other words they are very overpowered.
Here is a basic example when up against, knights:
If they convert a (full hp) knight it makes a difference of 270 resources that is you gain 135 resource unit value and enemy loses 135 resource unit value.
In addition a monk is useful to heal units besides collecting relics, it does save a lot of resources in a relative short time to heal a unit instead of creating new.
I don't know the exact healing speed of a monk but few tests I've done suggest it's somewhere between 2.1 and 2.5 hp per second.
Monks are too cheap for what they can do, my suggestion would be to double conversion time.
Here is a basic example when up against, knights:
If they convert a (full hp) knight it makes a difference of 270 resources that is you gain 135 resource unit value and enemy loses 135 resource unit value.
In addition a monk is useful to heal units besides collecting relics, it does save a lot of resources in a relative short time to heal a unit instead of creating new.
I don't know the exact healing speed of a monk but few tests I've done suggest it's somewhere between 2.1 and 2.5 hp per second.
Monks are too cheap for what they can do, my suggestion would be to double conversion time.