What I find interesting is, that it uses the logic of exponential decay. The more mana shrines you have, the less mana your spells will cost, but you can never actually reduce your spells mana cost to zero.
Well, you could, if you have enough shrines on the map to reduce the cost of spells to a value so small, that the floating point variable cannot contain it and goes to 0.0 .
If I calculated correctly, it would take 1987 = 1986 + 1 shrines to get to zero, using the double type.
Which is 1986 to get near 2.3*10^(-308),which is the smallest positive value a double type can hold, and one additional shrine to get below that.
If I calculte how much space 1987 3x3 shrines will occupy on the map, I get 17883 tiles.
For sake of simplicity, these would fit onto a 256x256 map, but would occupy a significant chunk of its 65536 tiles.
I doubt, that any serious mapmaker would consider stuffing his map with almost 2000 mana shrines, but it would be interesting to see wheter or not the game itself can handle this mana-madness.
Before I forget, will you be able to save multiplayer games?