[weby]

Economics of a fantasy world with significant number mages with the GURPS standard spell system would look totally different than any known fantasy world.

The ability to create ad manipulate huge amounts of materials(stone, earth,metals...) , the ease of canal building for transporting goods, the abundance of food with much less people needed as farmers, the ability to cure diseases, cure things like blindness and so on would make for a world that is totally different.

Thus trying to build logical traditional world based on GURPS magic is very difficult.

So I would not really worry about the math behind why something is something as it does not make sense in most cases.

If you want to use the standard magic system, then for creating magic items it is best to go the DFPRG route and just say "There is a guild that controls magic item creation, only they know those secret spells and they are not telling how much work time a given item takes their enchanters to create."

Otherwise you need to change and tweak a lot.