Fine, but this doesn't change the main point. Even if you figured it was worth your time to determine the effect of each candidate on your future, it still wouldn't compel you to vote for the candidate which helps you most; the expected value would still only be a tiny fraction of a penny, and you'd be better off using your vote to signal allegiance, assuage your conscience, etc.
...unless you are alturistic, and you value the impact on the other 300 million citizens (and the rest of the world). Then it can make sense to vote for who you think is best.
I think assuming that the only value you derive from voting being tax rate is the major flaw here.
Altruistic value is usually very high in the issues that people seem to vote based on: gay marriage, government health care, military operations, etc. Many people do value these things much higher than their tax rate.
The problem arises when we treat dollars as the unit of value. Value is not possible to practically quantify, and using money as the common denominator pushes things that actually are money to the fore, and fails to expose the things that do matter to people.
I discussed a tax increase simply because it made things simple, but you can assign a dollar value to any of the items you've mentioned. How much would you, personally, pay to legalize gay marriage?
$50,000? If so, repeat my calculation. $100,000? If so, then voting is worth $0.001 to you. $100 million? Then voting is actually worth $1 to you.
A much simpler explanation of voting behavior is the expressive value of voting. People devote great effort to cheering for their team and signalling allegiance - just look at sales of $50 Yankees pinstripes or $80 Paquiao jackets. Most people don't wait in line for 10 minutes just to donate $1 to a good cause.
What I'm saying, though, is putting a dollar figure on things like gay marriage is disingenuous. We have this conception that "value" automatically means that it can be put in terms of money.
Money has value, and events like law changes have value. However, both money and events have wildly varying value to different people. What a dollar is worth to me is different than what a dollar is worth to you, and indeed, what the millionth dollar is worth to me is far different than what the first one is.
The problem is this assumption that money is an appropriate expression of value. This is what I reject. It is an abstraction parallel to the bundles of mortgages that American banks were brought down by - putting it into a unit the MBAs and politicians can understand meant losing essentially all the meaning of what was bought and sold, and ignored fundamental fallacies in assumptions made.
Or, like most people, you're not a rational machine.
The future that costs you $50,000 (or helps/hurts you via any of the mentioned surrogate currencies) is distasteful. Your irrational brain won't let you do nothing, and makes you go out and cast your vote, even though you may understand that it's a waste of time.
Alternative theory: Risk.
Highly publicized close elections create a scenario that looms large in your mind: The Bad Guy wins by one vote. (Or just a few). You would never forgive yourself. Even though you know it to be fantastically unlikely, you aren't willing to risk it.
...unless you are alturistic, and you value the impact on the other 300 million citizens (and the rest of the world). Then it can make sense to vote for who you think is best.