Forcefield costs 12 mana to start (for 3 charges), which is 4 mana per charge (Same as 3 Blocks), and then costs 2 mana upkeep every round (for 1 charge).
If the forcefield is constantly soaking hits, it is amazingly mana efficient. If the forcefield sits at 3 charges for a while (for example, your opponent is killing your creatures and only using curses on your Mage) then its mana efficiency falls off.