Basically, If the following inequality is true, you should definitely vote:
K < 10 * S * V * (2c-1) * U/N
Otherwise, it might not be worth your time.
Variables:
Value of election outcome: V
Population of your state: N
Chance that your state is the tipping-point state: S
Confidence that you are making the right vote: c
Probability of an upset in your state: U
Cost of you voting: K