Spells cost the square of their spell level plus the spell level (min of 1) to cast.
So, a 0th Level cantrip costs 1 SP, a 1st Level spell costs 2 SP, a 2nd Level spell costs 6 SP, and a 9th Level spell costs 90 SP to cast, thus naturally limiting the number of high level spells any PC is going to cast, and heavily biasing the caster toward casting low-level spells.
The DM can also optionally limit the number of spells a caster can cast between rests, based on a factor of their ability scores.