Being in California, Gelman et al. put my probability of a decisive vote around 1/(5 million).

As the paper says:

[W]e consider how the results would change as better information is added so as to increase the accuracy of the forecasts. In most states this will have the effect of reducing the chance of an exact tie; that is, adding information will bring the probability that one vote will be decisive even closer to 0.

And as it turns out, conditional on polls and other information from right before the election, one would have to assign a very low probability that California will (almost) vote Republican. Also, conditional on California (almost) voting Republican, one would have to assign a very high probability that enough other states will vote Republican to make California's outcome not matter.

It seems to me that a reasonable probability estimate here would be multiple orders of magnitude lower than the cited estimate; and it seems to me that together with the optimal philanthropy point made by user:theduffman and user:dankane and user:JohnMaxwellIV elsewhere in the thread, this makes voting in states like California not worthwhile based on the calculation presented in the original post.