Reverse engineering the cost of a staff of power [PF]

[Carth]

Basically, I want to create a staff of power (http://www.d20pfsrd.com/magic-items/staves/staff-of-power), but with different spells in it. So I figure that the best thing to do is just figure out the price as though the default staff of power didn't have any special abilities, and the difference will be what I tack on to the cost of my custom staff. Following these guidelines (http://paizo.com/pathfinderRPG/prd/magicItems/magicItemCreation.html), however, I feel like I might have made an error somewhere. I get the following cost totals for each spell:
Globe of invulnerability: 18000 (6*15*400/2)
Hold monster: 11250 (5*15*300/2)
Cone of cold: 7500 (5*15*200/2)
Wall of force: 7500 (5*15*200/2)
Ray of enfeeblement (heightened): 15000 (5*15*200)
Lightning bolt (heightened): 15000 (5*15*200)
Fireball (heightened): 15000 (5*15*200)
Continual flame: 8500 (thanks to the material component, 2*15*200+50*50)
Levitate: 6000 (2*15*200)
Magic missile: 3000 (1*15*200)

That totals 106,750, with the actual cost to create being 117,500, a difference of only 10,750. Which seems odd, very odd, so I figured I'd double check to see if I went wrong anywhere?

[yougi]

Your calculations are a bit off: you always multiply by 15*5, but not all the spells are 5th level. Also, usually a crafter can decrease their CL to the minimum for casting the spell for magic items (on a wand or scroll for example). But I'm not sure if one can do that for a staff, as they use the user's level... Somebody else will have to weigh in on this.

You also assume a 400gp multiplier, while from what I read it's 375.

The difference in cost to create should be at least 16k: a +2 quarterstaff costs 8k, +2 luck bonus to save and AC are each another 8k, for a total of 24k, 12 to create. You then need to apply the +50% for the multiple abilities, although I don't know if they'd apply to the magic weapon's price... Either way, the sure will on the AC and saves bonus, so it's at least (8+(8*1.5)+(8*1.5))/2 = , before considering retributive strike and smite.

[Carth]

There are spells that are heightened to 5th level, which is why fireball, lightning bolt, and ray of enfeeblement cost what they do, as far as I can tell. Where are you seeing 375GP, I don't see that anywhere in the description of staff creation?