In decision theory, the "goodness" or "badness" of a decision is divorced from its actual outcome. Buying the lottery ticket was a bad decision regardless of whether you win.

However, don't forget that the value of money doesn't scale linearly with how much utility you assign to it. People tend to forget this. There is no rule that says you have to accept a certain $10 in exchange for a 10% chance at $100; on the contrary, it would be unusual to have a perfectly linear utility function in terms of money.

It's possible that your valuation of $5 is essentially 'nothing,' while your valuation of $1 million is 'extremely high.' If you'll permit me to construct a ridiculous scenario: let's say that you're guaranteed an income of $5 a day by the government, that you have no other way of obtaining steady income due to a disability, and that your living expenses are $4.99 per day. You will never be able to save $1 million; even if you save 1c per day and invest it as intelligently as possible, you will probably never accumulate $1 million. Let's further assume that you will be significantly happier if you could buy a particular house which costs exactly $1 million. If we take this artificial example, then it may be rational, or "a good decision" to play the lottery some fraction of the time, since it is essentially the only chance you have of obtaining your dream house.

e: In case the downvote is due to a belief that I am wrong in my assertions, I am prepared to provide citations and calculations to verify everything in this comment. Unexplained downvotes drive me nuts particularly when I know I'm right.