Can I use stat function to calculate expected value? I know when probabilities are equal the expected value is a simple mean but what about when probabilities are not equal? Can I still rely on the...

Yes you enter the probabilities as percentages in the Y registries.

For example, shares prices could be:

100p with probability 20%
110p with probability 45%
120p with probability 35%

Press 2ND 7 (DATA)

then 

X01 = 100, Y01 = 20
X02 = 110, Y02 = 45
X03 = 120, Y03 = 35

Press 2ND 8 (STAT) to access the expected value.