Or implement a global/national 'mana pool'...
Spells cost a certain amount of mana to cast, limiting the number each civ can use in a turn.
Summons are all permanent, but take a certain amount of mana as upkeep. Large initial cost, constant perturn drain, killed off if there isn't enough mana available for them.
Only
X mana regenerates each turn, increased by certain buildings/resources (maybe even by
untyped mana? Cost/benefit for gaining access to spells or leaving the mana unimproved).
Mana can only be accumulated to a cap of
Y (a cap keeps a civ like, say, the Doviello, from becoming arcane
beasts after suddenly gaining mages when they capture the Amurites. Without it, they'd have massive amounts accumulated), affected by buildings/traits/etc.
Tarq may recognize this idea.