Couple holding hands © Corbis

Everything is a numbers game—and marital infidelity is no exception.

According to a recent survey, the average affair lasts six months and costs $444 a month, or $2,664 in total. The survey, conducted by a U.K. retail company researching American spending habits, broke down the expenses incurred in a typical fling, such as hotel bills ($123), dinner and drink tabs ($162), gifts ($54), and date activities (movie tickets, $69, and "other," $36.)

But experts say the cost of having an affair is actually higher. Much higher.

"Secret cell phones, airline tickets, secret credit cards, hotel rooms—the costs can be astronomical," said Ramani Durvasula, a licensed clinical psychologist based in Santa Monica, California. "I know a woman and her lover who would book a hotel room to the tune of $2,500 a week. And keeping a mistress in an apartment—that can run in the hundreds of thousands."

Durvasula believes smartphones and social media have helped contribute to what she says is an enormous rise in marital infidelity.

"We've created the ultimate cheating tools," she said. "Social media is an accelerant. It's like giving children matches to play with. When you think about it, it's downright quaint these days to imagine a mistress calling a man's home and hanging up when his wife answers. Right? Now people can literally cheat on their spouses while lying right next to them in bed."

Durvasula says changes in the rhythm of a relationship—like a spouse who suddenly seems absent or detatched, or even overly involved—can be a clue that one partner is straying.

"It's not that hard to tell," she insists. "You don't have to be freakin' Magnum P.I."

Or maybe you do. Without hardcore evidence, many suspicious spouses seek help from a private investigator—a costly undertaking that usually leads to bad news, as Brad Robinson knows all too well.

Robinson, a senior partner at The Millennium Group Private Investigation Agency in West Palm Beach, Florida, says there's no shortage of players in the dicey, and pricey, game of sneaking around.

Robinson said many of his targets are trophy wives ("he's 70, she's 26; figure it out") or successful businessmen whose privileged status gives them "a sense of invulnerability."

"They think they're smarter than everyone else. They're so used to getting their way," he said. "These guys are constantly co-signing on loans and buying their girlfriends condos. We followed a guy who was taking his girlfriend to five-star steakhouses and taking his wife to Denny's. We watch guys go into jewelry stores and buy expensive jewels that far outshine the trinket the wife got for Christmas."

And expensive dinners and sparkling jewels might not even be the biggest drain on the credit card, said Robinson. The cost of covering one's tracks is a huge factor in calculating the price of an affair.

"Maybe you feel safer doing it far from prying eyes, but if you travel out of town it's going to cost you $10,000 at least," said Robinson. "We had a case years ago where a man hired us to follow his wife on a "shopping trip" to Paris. The husband paid us for everything—airplane travel, hotels, restaurants. Of course, when we got there, she was doing more than shopping."

Straying spouses should beware of Robinson, an ex-CIA operative, and his partner, a former FBI investigator. Their services include photo surveillance, video surveillance, GPS car tracking, hiring female decoys and even a forensics specialist who hacks the computer systems of suspected cheaters.

"A lot of our clients will say, 'I want you to follow him 24/7.' But we charge $100 per man-hour, so the first day will cost you $4,800."

All of this is pocket change, of course, compared to the price of getting caught.

Atlanta-based attorney Randall M. Kessler says infidelity is the top issue in divorce cases, and that the adulterer is usually the one who pays the "worst-case-scenario" price.

"[People who have affairs] have nightmares about being on the witness stand and having to recount their conduct," he said. "I've seen people pay a million dollars more than they should have paid, just to avoid the confrontation."

Overpayment to a wronged spouse is also a way to duck the messy aftermath of an affair, says Kessler, whose firm exclusively handles divorce cases.

"A man might wind up paying 75 percent of his income to his ex-wife because it makes him feel less guilty. And it's a way to avoid attorneys, avoid embarrassment and avoid getting the girlfriend involved."

Kessler adds that sometimes a person just wants to do the right thing.

"It's amazing how many people will say, 'Just give her the money. I don't want it.'"

After all the numbers are added up, however, the experts agree on one thing: The highest cost of infidelity is the emotional one.

"The betrayal of trust is the largest cost of having an affair," said Durvasula. "The guilt, the loss of self-esteem, the loss of self-worth—good luck putting dollars and cents on that."

Sheila Hageman, 43, a married writer and mother of three from Stratford, Connecticut, agrees. She says years of multiple love affairs destroyed her first two marriages.

"As a woman, I didn't have to put any money out, but still, the cost to me was incalculable," she said. "All the lying, the guilt, knowing I hurt someone I really loved—and still do love—I have to live with that the rest of my life.

"The cheating had nothing to do with him and with our relationship. It had to do with the pain I was in. I was looking to feel loved, to feel valuable. In the process I threw myself away."

Thomas Galiano, a social worker and self-help author from New Brunswick, New Jersey, who now counsels couples on infidelity, said he was able to repair his marriage after a five-year affair but that he and his wife still bear residual emotional scars.

"You destroy the trust in your marriage," he said. "Your children learn to keep secrets. They learn a distorted and warped idea of intimacy. At some level, the shame will always eat away at you."

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