As Mister K, but the first PP difference is -10 to the B.A.R./SCSM/Both table, then -5 for every PP fewer after that.
Also, the LEVEL of the SpellUser drops by -1 for every PP difference, also, to a Minimum of 1.
Wanna be a miser with PP? You can but then only gas fumes are coming out of that weak Fireball you’re casting, too. Not a problem vs ants invading your picnic, but laughable against that SL creature you’re hoping to whammy continuously while being conservative with Power Points.
Adopted that until finding thresholds for spending PP to cast spells worked better for my group and power level I wanted(1PP for all spells up to 5th LVL; 6PP for spell levels of 6-10th level, etc.)