In the Mega Millions multi-state lottery, jackpots are split equally among all winners who match all numbers.The Massachusetts lottery imposed purchase limits after it was discovered that their Cash Win-fail game had a positive expected value after a jackpot was rolled down to increase the payouts for lower level prizes.
The value of a large progressive jackpot is very sensitive to the exact rules of the game, so be sure to understand them.
Larger jackpots increase the payouts and therefore the value of a ticket.