Does anybody know how they come up with the prices for the metamagic rods? Cause I can't figure it out. I'm interested in looking into other metamagic adding items. Should that be allowed?
I can't figure the correlation between metamagic cost, max spell level and the price either. I've been messing with the data for the last hour or so. Getting an exponential regression for the prices isn't too hard, figuring out how that line correlates to the two variables (max spell level and metamagic cost).
The nice thing is that they have covered metamagic rods for +1-+4 metamagic cost at all three intervals (up to level 3, up to level 6 and up to level 9 spells). If you are needing to deal with metamagic costs of +5 or +6 (which only exist outside Pathfinder core) you'll have to rely on comparable items.
As to moving metamagicking from rods to other items, as a DM, I might allow it. It's going to cost 1.5 times the base rod price, for switching slots.
So, since this bothered me, I crunched a little more math on the subject. I didn't get any closer to figuring out a cohesive formula for calculating the prices of metamagic items. However, I did discover, oddly, that the metamagic cost vs rod price are linear regressions. They are perfect linear regressions for the regular and greater versions of the rods.
Within each type of metamagic cost, the lesser, regular and greater versions vs price are exponential regressions. Quicken had a perfect exponential regression, at 15767.9*1.3^x, where x is the max spell level (3, 6 or 9).
Someone better at the maths involved might be able to make sense of those facts, but I'd still probably have to just use the listed prices of the various metamagic rods and go from there.
Adjusting for rounding a bit, each rod of lesser, regular, greater are in factors of 1X, 4X, 9X. Each plus are in factors of 1Y, 3Y, 5Y, 7Y. The exception to this is the Lesser Quicken Rod, which is doubled in price (which makes plenty of sense to me).
Actual Prices:
to 3rd 3,000 9,000 14,000 35,000
to 6th 11,000 32,500 54,000 75,500
to 9th 24,500 73,000 121,500 170,000
Formula works out to (2*MetaPlus-1)*MaxLevel^2*300 - even with rounding that works out to within 200gp on all the regular and greaters.
Calculated Prices:
3rd 2700 8100 13500 18900
6th 10800 32400 54000 75600
9th 24300 72900 121500 170100
Personally, I think something more exponential on the MetaPlus would be more appropriate, or the adjusted max-level. I think the chart itself shows that the formula doesn't quite work, with the doubled price of a lesser quicken.
Perhaps: MetaMaxLevel^3*100
3rd 6,400 12,500 21,600 34,300
6th 34,300 51,200 72,900 100,000
9th 100,000 133,100 172,800 219,700
Tough call.