For a bit simpler system in my world, I have decided to streamline the expenses/costs for having items enchanted or having spells cast.
For each level of spell cast, the cost is 100 GP. so a 4th level spell is 400 GP. For multiple spells, say a 4th level spell and 5 first level spells, the cost would be 900 GP.
Cost can increase or decrease depending on the spell caster involved, how much energy is involved in casting the spell, etc.. For example, if the casting of a particular spell takes more than one day, there is an extra 50 GP per day added to the cost. Casting the spell "Permanency" costs a Magic User 1 point of Constitution from their total. CON points don't come cheaply and are valued of at least 2,500 GP each, maybe more, depending on the MU.
I give an XP value for the enchantments/spell casting at 100 XP/level of spell caster.