Rather than using strange godlearner tables, keep in mind a few things about
mercantile magics :
- supply and demand affect prices as everywhere ; if there's only one sorcerer
in town able to Shapechange Frog back to Human, he can name his price.
- often it's not money asked in payment, but service : if you do that for the
temple/shaman/wizard he'll cast the spell you need.
- Spell casters give priority to members of their community ; foreigners had
better be ready to pay a REALLY high cost, and pass some trials, before being
even considered worthy of recieving "our" magics.
- 20 silver coins (each about 5 grams) buy you a healthy cow or 7 sheeps.
- a specialist who is fed and lodged by his employer (such as a merc) gets
about 5 silver coins a week
- using all of these, make up what is the most interesting for you and your
players !
It seems your second question is : can PCs offer their POW for enchantments cast
by others (Armoring or Strengthening Enchantments for instance)? I don't think
the answer lies in the rules.
I'd say possibly yes for members of the same cult / organization as the spell
caster, especially fighting cults. But it costs TWICE THE NORMAL COST to supply
power from a different person as the spellcaster ; this will limit 50-hp
characters from happening too fast. And don't forget : if the PCs have it, so
can many NPC...