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While some Americans are concerned about the strength of the dollar and others are watching the international currency markets, a growing number are preoccupied with new currencies like Smufberries, Monopoly Gold, and Farm Cash. These and other kinds of "play money" are used in online games, but as some of the estimated 50 million players are discovering, it can pack an off-line wallop.

As anyone with a Facebook page or a computer-savvy child knows, there are now thousands of games to play via social network and mobile app, and it's common for many, including hits like Farmville, Tap Zoo, and World of Warcraft, to have their own currencies, which players use to buy any manner of virtual accessories. If an in-game pet is hungry, players can buy food; a vulnerable warrior alter-ego can buy additional armor. Individual purchase typically run around $5 to $10 a pop, but the bills can swell in a hurry. Game reviews are rife with complaints about hundreds of dollars in unexpected charges, and in one high-profile instance in February, a Rockville, Md., family reportedly received a bill for $1,400 worth of in-game purchases made by their 8-year-old daughter.

In the meantime, the government has taken notice, with Representative Edward Markey (D., Mass.) and Senator Amy Klobuchar (D., Minn.) asking the Federal Trade Commission to examine the games' marketing practices. "Companies shouldn't be able to use Smurfs and snowflakes and zoos as online ATMs pulling money from the pockets of unsuspecting parents," wrote Markey in a letter to the FTC. The FTC would not confirm or deny any ongoing investigation, but in a letter to Markey, FTC Chairman Jon Liebowitz said that the commission would "look closely at the current industry practice with respect to the marketing and delivery of these types of applications."

The game-makers failed to respond to repeated requests for comment, but industry defenders say that as long as developers are up-front about what a game offers and sells, responsibility lies with the players or their parents. It takes several steps to add and save credit card information so it can be used to purchase items in a game, and that's a deliberate action, says Michael Pachter, an equity analyst who covers technology companies for Wedbush Securities. "Everything is opt-in," he says. "It's not a hidden purchase."

And though the games may offer opportunities to buy virtual doo-dads and accoutrements, it's the platforms Facebook, Apple or Android that ultimately govern who can make purchases and how easily, says Reg Weiner, a vice president at Metaverse Mod Squad, a consulting group that works with social networking sites and developers.

At least one company has changed its policies. In early March, Apple made it harder to make in-game purchases; the company also promotes parental controls that can restrict in-game spending sprees. Google, which launched in-game payments for Andriod devices in late March, says it is considering similar controls but does not have them currently. Facebook says it allows purchases for any user, though a spokesman says they will continue to monitor the issue.

In the landscape of web-based games, many of which are commonly played on social networks like Facebook and via app on smartphones and tablet computers, spending virtual money is remarkably easy. The most popular online games cost little to nothing to start, but as players progress, the games start to offer enhancements that provide a strategic advantage for a price. In Monopoly Millionaires, a popular Facebook game, for example, players can pay extra for different kinds of houses or hotels, that charge higher rents; in a beta version of Oregon Trail, based on the popular 1980s computer game, players can pay for faster oxen, a nicer wagon or a better hunting rifle.

But to get the goods, players typically have to buy bundles of game-specific currency with real money via debit, credit or PayPal, though some games also give players the option to charge the purchase to their cell phone bills. So, for example, a Farmville farmer would pay $5 for 25 "Farm Cash," one of the official currencies of Farmville, with which he could then buy a Candy Apple tree (5 Farm Cash) or a Shetland Pony (24). For 120 Farm Cash, the equivalent of $24, he could get a Leprechaun Cottage.

Why not just charge in dollars? Having a unique system of credits, coins or other currency keeps players in the game, says Rodney Nelsestuen, a senior research director with Tower Group -- not unlike the loyalty inspired by airline miles or credit card points. Also, spending five gems to speed up the baking time for your virtual chocolate tarts in Bakery Story may be more palatable than spending the real-world $1 equivalent. "It seems like play money," says Edward Castronova, a professor of telecommunications at Indiana University.

For the game-makers, it's real profit. The game-makers now earn eight times as much money selling virtual goods as they do from advertising, according to research firm Flurry. And U.S. sales are expected to reach $2.1 billion this year, according to market research firm Inside Network more than double what consumers spent in 2009.

But this new model has roused all manner of frustrations, most of which have so far been contained to online reviews and comment boards. The most common gripes are from parents annoyed by surprising bills, but avid gamers also grouse that it's impossible to win a supposedly free game without buying additional enhancements. Because many of the games save credit card information and don't require a password or more than a few clicks for a purchase, it can be easy for a young player to make repeated purchases without a parent's approval or even knowledge. Until recently, a parent who thought he sprung for a $5 bucket of Smurfberries or gold coins may have found himself on the hook for $100 worth.

Capcom, the maker of Smurfs' Village, did not respond to requests for comment. But Apple, which sells that game, started requiring a password for every purchase in early March, closing the 15-minute window that used to allow for purchases without a password. (Of course, if a child has your iTunes password, this protection won't help much.) An Apple spokesman also noted that its new operating system for the iPad, iPhone and iPod has parental controls that can restrict in-game spending sprees.

But for disgruntled customers, it's unclear who's ultimately responsible, says John Breyault, the vice president of telecommunications and fraud policy for the National Consumers League. If a consumer wants to stop playing a game and cash in his credits, are the software developers on the hook, or is the platform responsible? In the case of the 8-year-old in Rockville, Md., Apple refunded the $1,400 in charges, but the company says that's not a permanent policy. That's a concern, says Breyault. "This is a Wild West area, and it doesn't seem like there's much oversight."

In the meantime, some parents are already taking steps to curb in-game spending. Castronova recommends buying in small amounts, and teaching your kids to translate how many real dollars and cents it takes to, say, cure Tobey's dysentery in Oregon Trail game. "Think of virtual currency the way you think about real currency," Castronova says. "If you decided to have yen in your portfolio, you'd track its value." And if possible, don't link a credit or debit card to an account for automatic purchases; for sites that require it, like iTunes, don't share the password with your kids -- or link a prepaid card with a limited balance.