Re: spells and active cost
Because the power of the Dark Crystal (OAF) focuses the magical energies, making them easier to control? I don't necessarily agree with his take on magic, but its easy enough to explain this stuff away.
As for the OP, maybe you can just create your own system for calculating Mana Use and RSR for your game. Something like...
Mana Use/RSR Modifier
For this magic system, calculate Mana Use/RSR normally. Then, take the result and apply it to the following chart...
Revised Mana Use/RSR Modifier
If Limitations = 0.00 - 1.0, then Normal Mana Use/RSR Modifier * 1.00
If Limitations = 1.25 - 2.0, then Normal Mana Use/RSR Modifier * 0.75
If Limitations = 2.25 - 3.0, then Normal Mana Use/RSR Modifier * 0.50
If Limitations = 3.25 - 4.0+, then Normal Mana Use/RSR Modifier * 0.25
So, a standard 50 point power, no limits, still costs 5 Mana and -5 to RSR.
50 pt power with -2.25 in lims, would be 5*0.5 = 2.5, Hero rounding making it 2 Mana and -2 RSR.
50 pt power with -5 in lims would be 5*0.25 = 1.25, or 1 Mana and -1 RSR.
Obviously, you can set the break points where you want. Its an extra step, but a pretty easy one that should approximate what you want, I think.