If you're a player, the formula is "ask the GM." If you're the GM, compare the proposed staff to existing staffs and make an estimate.
To my knowledge there are no established formulas as of yet. Perhaps someone could reverse engineer a few and try and determine a pattern?
The forthcoming Gamemastery Guide will be providing guidance for such things, though not exact plug-and-play formulas. Until it comes out (Jan?), what Ravingdork said.
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With exceptions, the 3 general rules I have determined are:
1) All spells in the staff share a trait, sometimes 2. e.g. Necromancy, Plant, Light, Positive
2) Level is 1 greater than it would be for a wand containing the highest level spell.
3) Cost is between 1.2 and 1.5 times the cost of a wand that would contain the highest level of spell. With a mean of 1.33.
The exceptions noted so far are:
1) The basic staff of flame, which has the same level and price as a wand for the highest level spell. I suspect this is a mistake / errata.
2) The animal staff, where spells don't necessarily share traits, just a common theme.
3) The two very high level rare items: staff of the magi and staff of power. These both break the trait sharing "rule" and the staff of the magi costs 2.25 times the cost of a wand.
It looks like "rare" should cost at least 50% more. And therefore I suspect the staff of power is missing a 50% rare cost uplift.