What\u2019s not to like about something that\u2019s free? Turns out, plenty.\n\tThe old adage \u201cnothing in life is free\u201d has taken on new meaning in today\u2019s mobile era. Smartphones power a bounty of mostly free apps \u2013 some one million apps have landed at app stores since smartphones emerged on the scene five years ago. Android and iPhone users download free apps like M&Ms, thinking there\u2019s no downside.\n\tBut free apps can cost a lot.\n\tFor smartphone owners, this cost comes in the form of battery life, according to Purdue University. In a study released earlier this month, Purdue researchers found that free Android apps spend up to 75 percent of their energy tracking geographical location for ad targeting, sending user data to advertisers and downloading ads.\n\t\u201cIt turns out the free apps aren't really free because they contain the hidden cost of reduced battery life,\u201d says Y. Charlie Hu, an electrical and computer engineering professor at Purdue University.\n\tPurdue researchers analyzed six free Android apps, including Angry Birds, Facebook and Android Browser. Angry Birds, in particular, used 75 percent of its power consumption on running ad modules and 25 percent on the game play.\n\tPoorly written free and paid apps can also be terribly power inefficient. For instance, one free app failed to turn off its connection to the Internet, according to the Purdue study. Power-efficient apps have a power-usage spike when sending data over 3G and then lower power usage within a fraction of a second, but Purdue found some free apps continue draining power for about seven seconds.\n\tBattery life aside, free apps also can persuade \u00a0smartphone owners into in-app purchasing. After all, most free apps are teasers enticing owners to upgrade to the paid version.\n\tFree apps are definitely not cheap for tech companies, either. This week Facebook bought Instagram, a free mobile photo sharing app, for a whopping $1 billion.\n\tClearly, these companies offering free apps are making more money from us than we know. Instagram\u2019s value skyrocketed, which is why Facebook wanted to staunch Instagram before it could became a real threat and to make sure it didn\u2019t fall into the hands of competitors, such as Google and Apple.\n\tNevertheless, a billion dollars for a free app \u2013 nothing free about that.