It's a trusted formula: Send your sweetheart flowers, chocolates or teddy bears on Valentine's Day to express your love.

But what about sending a $10 image of flowers, chocolates or teddy bears to someone's online profile?

How about spending $50 on an icon of a pile of cash to impress a stranger you want to date?

The concept of paying real-world money for a virtual good has been around for a few years in games and social networks. On Facebook, for example, users can pay a few bucks to decorate a friend's profile page with an icon -- like a beer mug or a birthday balloon.

But in the past few months, several online dating sites have introduced virtual gift stores -- and users are jumping at the chance to woo a mate.

As mystifying as it may seem to traditional romantics, virtual gifts have become a $1 billion industry in the United States according to the research group Inside Network. Spending money on virtual goods has been huge in other parts of the world -- analysts estimate it to be a $5 billion industry worldwide.

Snap Interactive in New York runs the Facebook application ``Are You Interested?'' where users can find potential partners and send them virtual gifts as an ice breaker. The publicly traded company introduced a new gift store about two months ago, and CEO Clifford Lerner said the application is raking in ``a couple thousand dollars a day'' just from gift purchases.

More than 2.4 million people use the application monthly, the company said. Gifts start at about $1.25 for a heart and $1.75 for a rose, and get more expensive the more unique or animated they are -- like $20 for a picture of an engagement ring or gold bricks, or $10 for a teddy popping out of a gift box with the words ``Be My Valentine'' appearing on the screen.

Lerner said introducing a $20 icon of gold was just a test to see if people would spend the money. And they did. Four were sold within the first hour.

``We really believed virtual gifts would add a lot to the site and we had a lot of interesting ideas,'' Lerner said. ``It blew our mind that someone was going to spend $20 for a bar of gold, let alone four in the first hour.''

So the site offered more expensive icons based on the demand, like ``I'm Rich!'' which shows a pile of gold, cash and jewels, and ``You're Priceless!'' which is a guy holding a suitcase full of cash. Both sell for $20.

HIGHER PRICES

In the world of online flirting, virtual gifts sometimes defy the normal laws of supply and demand.

``We've actually seen a lot of times that the higher cost of some of these gifts, the more we actually sell,'' Lerner said. ``If a girl is not interested in a guy, and she sees he spent $20, she's probably going to respond. And that's worth it to the guy, just to get a response.''

Dr. Eva Ritvo, vice chair of the Department of Psychiatry and Behavioral Sciences at the University of Miami Miller School of Medicine, said sending a stack of cash icon that costs $50 is just another way of a guy showing he can provide for the woman.

``It's another symbolic representation that I have money and I will be able to take care of you,'' Ritvo said. ``It's a modern day equivalent of men going out for the kill.''

She adds that the value in a virtual gift would be high for someone who puts a lot of time and energy into their online profile.

``If someone is very into the virtual world and spends a lot of time on Facebook, sending an icon is speaking their language,'' Ritvo said.