First, try and see if <Value>-10</Value> works. I suspect that, assuming you can have negative values for this modifier, this money loss will be affected by the AI tax rate, since it will be applied to city income.
Next, for a one-time bonus per spell cast, you might be able to use the code from Alchemy, or you might be able to set a negative resource cost (e.g., casting the spell "costs" -10 gold). I don't know if spell costs can be negative, though, and I don't know that a spell can have multiple target types.
For a multi-turn spell, you could try setting a negative upkeep value for the spell and see what happens.
I don't know if any of this will work, however.