Different sets definitely have different average booster values, and you can calculate the current value of opening a booster relatively easily. You have a 1/8 chance of a mythic, and a 7/8 chance of a rare, so you need the average rare price (ARP) and the average mythic price (AMP). These values may not be directly available, but are trivial (if time consuming) to calculate.
With those values, you can work it out as follows:
(((ARP) / 8) * 7) + ((AMP) / 8).
This might give slightly too high a value as you cant get a rare AND a mythic in a pack, but it gives a reasonable representation.
For more accuracy, you can also factor in the value of a foil (the chance of a foil I believe is 1 in 7 packs, the probability of any individual foil I believe is 1 in the number of cards in the set, however both of those may be incorrect, I'm sure someone else can correct that).
The average value of the uncommons can be factored in (you get 3 in a pack) and if you REALLY want, the average price of a common also.
all of this can give you an estimate, but for someone buying boosters its of little use. Its valuable for vendors to calculate these values as they can figure out how to value buying and opening boosters to sell singles, but only because they can open large supplies of product to realize these averages. An individual will not be able to buy enough.