It's simple mathematics people: A copy of PopSci costs $8.95 on the newstand. A 12 month subscription costs $84.95, which works out at a saving of $22.45 over a year. A copy of Spore will set you back about $90. Which means that if you subscribe to PopSci this month, you'll end up getting $112.45 worth of free stuff for your 85 bucks.
Not only that, but you can enter the mag's other competitions, which include the chance to win a $900 Blu-ray pack, a $900 Tamron lens and a $140 STM bag. That's a lot of value for your purchase, right there.
The timeframe for the free Spore giveaway probably ends fairly soon, so you should get subscribing sooner rather than later...