I've thought about this a lot, and to me, the question boils down to the average expected loss. Once a jackpot increases to a certain level, the average expected loss is actually positive. In your example, assuming the tickets cost $1 each, then the jackpot would need to be just shy of $1 million for me to be willing to play.

In the USA with the Powerball, this number (with a $2 ticket) ends up being around $600 million. When it goes that high, I buy a ticket, because the average expected loss is >$0.

I do know there is one major flaw in my thinking: I am not accounting for the very likely outcome of a split jackpot between multiple winners. What can I say? If my inner child wants to have some fun and ignore one of the variables, who am I to stop him? :P