Oh boy, I misread this spell completely. Given that it only affect province income, it's actually not overpowered IMO. Still useful though.
Some calculations:
Cast at the minmum level (9th) it affects two provinces for 3 turns. Casting it on a level 6 and 5 province, with Heavy taxation and full Law collection, will have the following effect:
Income (level 5): 5*5/5(base income) *0.5(multiplier) *0.4(law collection) = 1.0 GB
Income (level 6): 5*5/5(base income) *0.5(multiplier) *0.4(law collection) = 1.4 GB
Over three turns, that adds up to 7.2 GB (some rounding error here)
The spell costs 5 GB to prepare and 4 GB to cast, meaning it looses 1.8 GB, but it also costs 16 RP to cast, which is not covered.
However, it also adds 5*2*3 = 30 GB in province growth, which is a hefty bonus.
If we boost the casting level to 11, affecting 3 provinces (adding in a level 4 province) for 3 turns, the following happens:
Income (level 4): 4*4/5(base income) *0.5(multiplier) *0.4(law collection) = 0.6 GB
Income (level 5): 5*5/5(base income) *0.5(multiplier) *0.4(law collection) = 1.0 GB
Income (level 6): 6*6/5(base income) *0.5(multiplier) *0.4(law collection) = 1.4 GB
Over three turns, that adds up to 9 GB (some rounding error here)
So now the GB cost breaks even, but there's still the 24 RP. However, now the growth bonus becomes 5*3*3 = 45 GB.
So the spell's purpose, as it is now, becomes clear. It is NOT for gaining an income, it is for boosting province growth. Unless we're either talking very high caster level (12+) or very high level provinces. That leaves a few possible issues:
Is this the intention of the spell? If not, something needs to be done.
The calculations are kinda difficult to implement in the P&H, though some small changes can be made to the P&H to make it easier.
For the suggestions:
It is kinda weird that is has no effect on Trade Holdings. Since trade explicitly represents income from export/import, that might be preferably.
Giving a constant bonus would make the spell easier to predict and make it less powerful for high level provinces. An option could be to add +5 to the province level for the purpose of Province collection only... this would open some funky questions - mainly if this stacks with Bless or not and how on earth to calculate it. Adding directly to the province level, like a bless spell, would be very powerful. I think this should remain the domain of the bless spell.
Calculations:
+5 to province level for the purpose of province collection only, heavy taxes, level 4, 5 and 6 province:
Level 4: Income: (9*9)/5*0.4-(4*4)/5*0.4 = 5.2 GB
Level 5: Income: (10*10)/5*0.4-(5*5)/5*0.4 = 6.0 GB
Level 6: Income: (11*11)/5*0.4-(6*6)/5*0.4 = 6.8 GB
Err... Broken. Minimum level (on 4 and 5 province): +33.6 GB from one casting. Inflation! Bonus need to be smaller. +2 bonus is the absolute maximum for this bonus, anything above gets crazy.
So, the questions remain:
Is the purpose of this spell to boost income or to boost growth? If the first, it needs to change, if the latter, it's fine.
Should it affect Trade Holding levels as well?
Should the effect be changed to make it easier to implement in the P&H or should the P&H be changed slightly (as we go, starting with the provinces affected) to make it easier to calculate?
Why is the pie always a lie?