It says the spell doesn't get the full item value but I can't find the info about its exact mechanics.

Can it serve as replacement of packmule(s) usage?

Well, I started a new DS1 LoA playthrough with dual mage character with no party and no mules. Transmuting stuff into gold doesn't seem like very lucrative business so far. Does Transmute get better with Nature magic leveling up? And does Transmute usage add to Nature magic level grow?