Jokes are a strange currency. Their value rests on shared agreement, and that value can increase or decrease over time. Jokes can saturate the market—think double entendres on Anthony Weiner’s name—and react to supply and demand. They can be stolen, but it’s not recommended. And for most comedians, they don’t exactly put food on the table—unless you’re cool with ramen.

So jokes have worth, but it’s safe to say comedy isn’t a get-rich quick endeavor. I’ve been performing in and around New York City for just over three years and I estimate I’ve paid $500 to practice my jokes at open mics, usually $5 a pop. That may seem like a lot, but for a comic developing new material, it’s a worthwhile investment. It’s taken me all three of those years to assemble 10 minutes of material I feel good about telling. Whenever I do make a few bucks from the door or the tip bucket, it feels like I’ve won the Powerball.

On any given weeknight, professional stand-up comedians—people who have appeared on late-night television and gets thousands of dollars to headline shows—can be found in any New York comedy club, telling jokes for free. There’s no way to put a price tag on the exhilaration of making a group of strangers laugh…but I asked some comedians to try anyway.

Jokes are an investment

Comedians don’t just get up on stage and “be funny.” Each joke is honed, agonized over, and refined until it’s ready for a paying audience.

“If I can get a joke to work on [a free] show, that joke is worth money,” Mark Normand, a stand-up comedian who has appeared on The Late Show with Stephen Colbert and Inside Amy Schumer, told me in April. “A joke is currency. If you do a Conan set, you get a couple grand, and that’s 5 minutes of jokes. So if I can get a joke to work, to me, that’s as good as money.”

Comedy legend George Wallace has been telling jokes for 30 years on the road, on late-night, on HBO, and up until recently at his sold-out shows at The Flamingo in Las Vegas.

“I’ll perform for free or for next to nothing on a Monday because that’s when I’m trying out the new jokes,” Wallace says. “I’m up there with my notebook and everything.”

Of course big-ticket weekend and corporate shows are a different story. “Those people expect a polished performance,” Wallace says. “They also get what they pay for.”

Jokes have lasting value

Just how lasting? Depends on who you ask. Louis CK reportedly tosses all of this material after a year. Other comedians, like Jackie Mason, are happy to trot out the same chestnuts as long as the laughs keep coming.

Comedians with decades under their belts can bring back jokes from years past, retrofitted for modern times. Wallace, who says his “brain is like a computer full of jokes,” is one of those.

“A good joke, you can get six months out of that, depending on the audience,” he says. “Florida, for example: It messed up two elections, Marco Rubio and Jeb Bush are having trouble getting re-elected, Trayvon Martin…it’s never-ending. You can just build on it.”

Of course, non-topical jokes can also last for as long as the comedian and audience enjoy hearing them.

“If it’s not based on timely events [and doesn’t] require a certain proximity in terms of your age or experience when you tell it, some jokes remain evergreen,” says Aparna Nancherla, a New York-based comedian who recently had her first half-hour special on Comedy Central. “Some people can keep telling the same jokes for years.”

…until they don’t

Ask any comic: A joke can die without warning.

“It’s a weird thing. You’ll have a joke that used to kill and then one day, it starts to get nothing,” says Sean Donnelly, a comedian who has appeared on Comedy Central and Last Comic Standing. “Maybe you’ve become bored of telling it over and over and you’ve given up on caring about it. The audience isn’t having fun listening to it anymore.”

Some of that is a natural statute of limitations: We only have a few weeks (or months) of “winning” in us before it and other punchlines about Charlie Sheen’s tiger blood start to put us to sleep. The same goes for subjects like Caitlyn Jenner, or Dick Cheney’s hunting mishaps.

“I’ve got no problem doing an old joke,” says Andy Kindler, comedian and author of the “State of the Industry” address at the Just for Laughs comedy festival in Montreal. “But my Y2K material isn’t going to work anymore. It’s just not timely.”

A joke doesn’t have to be ripped from the headlines to suddenly feel like yesterday’s news. Comedians have their own rules about when and where a punchline can be repeated, especially when it comes to late-night TV.

“I wouldn’t necessarily do the same jokes I told on Letterman on another late-night show,” says Kindler. “But those jokes will definitely still work on the road for a long time.”

Jokes are ownerless…technically

On an episode of HBO’s Curb Your Enthusiasm, comedian Richard Lewis agonizes over the fact that Bartlett won’t credit him for coining the phrase “the ____ from hell.”

“Richard started that: the date from hell, the mother-in-law from hell,” says Kindler. “But suddenly everyone on stage was saying it.” In 2006, Lewis was given more formal credit for the phrase in the Yale Book of Quotations, but comedians today are still saying “the set from hell” or “the Tinder date from hell” with little concern for attribution. Which speaks to one of the most irritating thing that can affect a joke’s value: another comic telling the same one, or a close facsimile.

“Whoever is doing it better, that person gets the joke,” says Donnelly. “If they’re more famous than you, whoever did it on TV first, they get it.”

Enter social media: a great way for comedians to gain a large following, and also for the distribution of stolen material, and the airing of grievances over that theft. In 2015, Josh “The Fat Jew” Ostrowski caused an uproar by posting other comedians’ jokes and memes to his Instagram feed unattributed. TBS’ Conan was slapped with a lawsuit last year for allegedly using another comedy writer’s tweets in a monologue.

Adding insult to injury, accusing someone of theft doesn’t restore the value of the joke: Now the story is the stealing itself. The effect of the joke is lost.

But what if they’re goods?

Normand may consider his jokes to be currency, but Yoram Bauman, PhD, a stand-up comedian, economist, and author of The Cartoon Introduction to Economics wonders if they’re better classified as goods.

“Jokes are not unlike other public goods, such as fresh air, street lights, an unobstructed view of Mt. Rainer,” he says. “If one member of the audience enjoys a joke, it doesn’t preclude anyone else from laughing at it.”

But even estimating the value of a public good has given economists headaches for years. (Not everyone puts equal value on having a nice view, or being able to see the ballet, or hearing a well-crafted punchline.)

Jokes, like goods, can also suffer from market saturation. If a showcase of eight comics includes eight separate bits about Donald Trump’s hair, or Hillary Clinton’s pantsuits, the audience will ultimately refuse to “buy” those jokes, by not laughing or, worse, heckling.

Even though most standup comics aspire to make a living from comedy, their primary chase is for laughs and fans, not cash. All are thrilled to be making any money doing what they love. And how does one put a number on something that brings personal and creative fulfillment?

“Comedy is my sex, my drugs, my rock and roll,” says Wallace. “I’d do it for nothing.”