Two weeks ago, we noted that Mega Millions was becoming a good bet. With no winners since, the jackpot has become much richer -- it's now estimated at $640 million. So you should buy tickets, right?
Not quite. To put the odds in your favor, you want a big jackpot and a relatively small number of tickets sold for the current draw, so if you win, you're not splitting the jackpot with one or more other tickets. The last few draws would have been an excellent time to buy tickets: a $363 million jackpot most recently ($261m cash value), and not a lot of tickets sold for each draw. No one else hit, so if you'd bought the winning ticket, you'd have kept it all to yourself.
You can get a rough idea for the number of tickets sold in each draw by looking at the change in jackpot size. The current cash value is $462 million at the time of this writing (and likely to go even higher by the time of the draw tonight). The cash value is 72% of the headline jackpot (this used to be around 50% before Zimbabwe Ben destroyed the time value of money with 0% interest rates, but that's another story). So you can back into prior jackpots' cash values by multiplying the headline jackpot by 0.72.
The current cash value of $462 million is an increase of $201 million over the last drawing's $261 million. So how many tickets have been sold? I haven't seen the exact payout percentage for Mega Millions, but most lotteries, including the California Lottery which participates in Mega Millions, are very close to 50%. And while Mega Millions has smaller prizes, the vast majority of the payout is in the big jackpot. So a pretty good approximation is that for every $1 increase in cash payout size, two $1 tickets have been sold. The $201 million increase for this drawing implies that around 400 million tickets have been sold. 400 million tickets in a 1-in-175-million drawing means there's a pretty good chance two or three other people already have winning tickets. After taxes, if you win, you would very likely end up with less than $100 million.
That's a losing bet.