The full caster class gets level+1 spell points. The spell cost is the slot itself. The highest slot possible is half the max spell points. So a level 5 caster has 6 points, can cast upto slot 3 spells, and Fireball costs 3 points. The spell points refresh after each rest (short or long).
Nifty! I like this approach and am going to look at using it in my games. I like that it makes spellcasting more of a menu of focused options. If I keep it to simple spells, I might even be able to summarize spells within the stat block.