The price of the item doesn't need to be figured out.
Having an NPC cast a spell under the services rules is
Caster level × spell level × 10 gp
The price of a scroll is
Spell level × caster level × 25 gp
Now, invisibility is a 2nd level spell. If I was capable of casting the spell myself, odds are that it would be free. If I need a scroll for some reason then the price would be 2×3×25=150. From an NPC source it would be 3×2×10=60.
Permanency has some different requirements beyond just needing to be cast. In this case invisibility costs 5,000 gp in base resources. To obtain the scroll would be 5×9×25=1125, and the NPC 9×5×10=450.
The following prices do not include the cost of the item to make invisible.
To do the job yourself should be fairly straight forward and easier to manage (if not a bit more costly) at 150 scroll + 1125 scroll + 5000 materials = 6,250 gp.
To get the job done with an NPC would require a large city due to permanency being a 5th level spell. 60 for invisibility + 450 for permanency + 5000 for materials = 5,510 gp (a savings of 740 for some inconvenience).
Do I have the prices correct or have I missed any steps?