A seat of the pants method is, you calculate the number of possible combinations for the lottery in question, and when (jackpot value) > (cost of ticket) * (# of combos), it becomes theoretically +EV. You should take into account whether it's a lump sum or an annuity (the annuity is worth very roughly half as much as the stated value of the jackpot), and if you really want to be thorough, find out the odds of winning a lower-tier prize and factor that in (more bother than I care to go through).
I play occasionally, when the jackpot > $100,000,000. At that point, I get enough entertainment value from fantasizing about winning to be worth dropping a few bucks. I know it will almost certainly never happen, but it's a lot of fun to dream about. [img]/images/graemlins/smile.gif[/img] I picked the $100,000,000 jackpot size as a nice round number that takes a while to reach and doesn't last long, so it automatically limits the $ I waste on this silliness. [img]/images/graemlins/grin.gif[/img]
Which reminds me, I want to swing by the store on my way home and pick up some Powerball tix tonight and see if I can start my Vegas vacation a week early.... [img]/images/graemlins/laugh.gif[/img]
-Mike
P.S - Here's a link discussing how to figure out the number of possible combinations in a given lottery:
Ask Dr. Math.