Spending a dollar on a lottery ticket doesn't have to be a waste of money. If you get a little bit of excitement from it, maybe it was the best use for that dollar. But why do people get more excited when the prize gets bigger? In what way would winning $640M be better than winning, say, $10M, or even $1M?
The odds of winning the Mega Millions lottery are 1 in 176 million, so I suppose someone might say that it makes sense to play anytime the pot exceeds $176M. That's an interesting thought. If you had the money and bought 176 million tickets you'd have a pretty good chance to win, but that would be far from certain. And if you had bought those tickets, you might have been one of the 3 winners in this lottery and won $213M and change, but since you were already a multi-millionaire you probably wouldn't be really excited about it.
My thinking is that the difference between winning $640M and $1 is that the big prize would be at least 640 times more likely to ruin your life. I'd much rather have 640 chances to win $1M than 1 chance to win $640M, but I wouldn't pay for them.
Lottery officials know that people jump up with their wallets open when the prizes get big, which is why the odds are so steep. But if they changed the game so the odds were about 1 in 1 million to win $1M, I think they'd make more money. Maybe not right away, but before long you'd hear about people in your city who won, or maybe even someone you know. Then even I might head down to the liquor store to buy a weekly ticket. Even though I'm not likely to win anything with a million to one odds, it would be more exciting than having a ridiculous chance to win a ridiculous amount.

