It is not 50/50
If there is a limited amount in the envelop, there are chances only pair of (0.5X, X) exists, and pair of (X, 2X) never exist.
with limited amount(L) in the envelop,
no matter where the X lays between 1 and L, 0.5X is always be there....100% chance of having (0.5X, X)
if X is greater than 0.5L, there is no 2X...50% chance of having (X,2X)
so for an unknown X, EV of switching = 100%/150%*-0.5X + 50%/150%*+X = 0
without limited amount(L) in the envelop,
for any X(X still less than half infinity anyway), EV of switching = 50%*-0.5X + 50%*+X = +0.25X
X=$100,
for a limit of $200 or higher, the pair of ($100, $200) is possible, switching EV=+$25
with limited amount less than $200, the pair of ($100, $200) is impossible, switching EV=-$50