I consider voting pointless according to my "utility function," if I am measuring the benefit to society that results from the fact that I voted for a particular candidate, basically because I have to take into account 1) the probability that I am mistaken about the better candidate, and 2) the bounded character of my utility function, which means that a small probability of affecting the outcome really does mean small total utility.

Given those facts, if I choose from that utility function alone, from a baseline position I would not vote at all, and I would be willing to vote for any candidate, including Trump, for a relatively small sum of money.

However, I am not a utilitarian in the first place, and apart from that, even if I were, I would have to take into account the effects on my character, as in your argument.