How much is a mining pool share worth on average? I know that mining pool shares have no value on their own, but it should be possible to find an average value based on the pool's average payout per share. This site describes the expected payout per share as ([block reward] - [pool operator's cut]) / [difficulty].

It seems to me that a share's "value" would necessarily be related to the difficulty as set by the pool--a pool with shares that are harder to find will have its users find fewer shares, making each share "worth" more. However, the equation above clearly doesn't work with the pool's difficulty, as most pools set difficulty to 1 and a share isn't worth ~48 BTC. How can I reconcile these issues to calculate the expected payout per share of a pool without knowing how many total shares are found?

Edit:

Ah, I think I've figured it out. The difficulty in the equation can be the network difficulty, because that's what makes it more or less likely that a given successful hash (earning a share) in a pool solves the block and earns the block reward. Thus, the network difficulty indirectly limits the value of a share. Is this correct?

Edit Again:

And now I'm back to being confused. If Pool A sets a higher difficulty than Pool B, Pool A's shares are worth more than Pool B's (assuming the same number of miners at a set hash rate) because they're rarer so each share will earn more at payout. How is this taken into account in the equation?
If the global network difficulty is D and the difficulty of shares in the pool is d, then the probability that a share will lead to a valid block is d/D. The reward in this case is B, so the average reward per share is B*(d/D). If the operator's average fee is f (so for example f=0.01 means 1% fee), the average reward miners get per share submitted is (1-f) * B * (d/D).