Switch able weapons?

[Donny Brook]

Do you want price in Character Points or money?

[Ulzgoroth]

Fundamentally equipment costs (in terms of money) aren't balanced as a rule, so the real question is what it needs to cost for your setting, not what it 'should' cost in some abstraction.

That said, things like Psychotronic Generators do put a systematized dollar value on what's essentially a Gadget.

And with a little wiggling Low Tech Companion 2 would cover a Naginata with a full-size added spear head...not that there's much benefit to that pairing.

[Flyndaran]

Even the most basic ubiquitous items and services aren't very stable between places now. Just look at one bedroom apartment rent for any dozen cities in the U.S.
Things are less stable in low tech societies, and once you bring in magic, things lose nearly all semblance of easy predictability.