Help finding an old concept; natural RAW result being magocratic city-states

[awa]

its not based on magic item creation but on traps

traps can be made to cast a spell every time they are activated at a cost exponential cheaper than a magic item granting the same effect.

I dont have the formula but as a simple example a inflict light wounds trap is 500 gp. Switch it to cure light wounds and you have a 500gp (maybe 1000 if it isn't market price) room of infinite healing.

https://www.d20srd.org/srd/traps.htm#magicTraps