Not that I think you'll make a rational decision about it, of course, but the most likely result, by a significant margin, of playing all 15 mega balls is that you'll lose much more than if you buy just 1 or 2 tickets. That means that doing it would be really stupid unless you've got more money than you need.
The lottery keeps 50% off the top and almost 38% goes to the top 2 prizes, paying out only 12 cents of every dollar for prizes with odd better than 1 in 750,000. Unless you spend well over 100 grand you'll be very lucky to win back more than 12% of what you spend. The flip side is that you will almost certainly lose at least 88%. Spend $104 playing every drawing in a year and you should expect to lose $91. Spend $1040 and you should expect to lose $910. Spend $1040, win 7 times as much as probability suggests, and you'll still lose almost twice as much money as if you had just spent $104.