“ That means the expectation value based on the ensemble mean of the first coin toss is (0.5x$50 + 0.5*$-40) = $5”

This reasoning is wrong. This is not the expectation. In fact, the expectation is zero. Please see: https://medium.com/@mikeharrisNY/gamble-expectation-and-the-ergodicity-conundrum-ffe3a523347b