The expected value of the points earned on a single roll in the game would take into account the probability of the numbers of the cube and whether or not they're prime. If the cube is a dice with numbers 1 to 6, then the expected value of the points would be:
P = 1(1/6) + 2(1/6) + 3(1/6) - 4(1/6) + 5(1/6) - 6(1/6)
P = 1/6 or 0.1667