I agree that it's not wise to ignore the associated opportunity costs, but it's a rather common fallacy (at least, one that's popped up quite often here) that one's time is fungible for money at the rate one is compensated for work.

On the other hand, for many individuals there are also likely to be associated gains, such as the fact that voting tends to be widely viewed as an effective signal of conscientiousness. Personally, whatever my feelings about the likelihood of my vote having a meaningful effect on the course of an election, I would prefer most of my acquaintances to think of me as the sort of person who votes.