Irrational exuberance is so widespread in hip-hop right now that Alan Greenspan might freak out — provided he understands rap lyrics at all, that is.

There's no two ways about it: Hip-hop sales stink. Album sales dropped 30 percent in 2007, a figure that includes digital downloads. Ringtones, which have given folks like Soulja Boy a reason to, um, soulja on, are not a big enough business to be an industry panacea.

But from recent rap songs, you'd think these guys were too rich to even stand up. Literally. In fact, Fat Joe's recent single, "The Crackhouse," begins "I'm sleeping on a billion dollars." This from a guy whose last CD, Me, Myself & I, sold about 250,000 copies.

Once upon a time, MCs were content to exaggerate their wealth in somewhat plausible ways. Only a few years ago, in fact, T.I. made the fairly modest claim to be worth a "couple hundred grand" in "Rubberband Man."

But those days are ancient history. Nowadays, even the lowliest rappers claim Steve Jobs-type paper stacks. Perhaps the most egregious offender is Bow Wow, who claims on his recent album that "[i]t's like every time I breathe I make a million."

Seriously? Human beings take at least ten breaths per minute, so that puts him at about $600 million an hour, perhaps double that if he's dancing. And if he really makes that much money, his record company must be pissed, considering that his latest album, the Omarion collaboration Face Off, debuted at No. 11 on the Billboard 200 and likely won't go gold.

R. Kelly, on the other hand, is a bona fide megastar and probably quite rich. Nonetheless, if only because of his current mountain of legal fees, it's hard to believe him when, guesting on Beanie Sigel's "All of the Above," Kelly claims, "I'm worth about a billion, but I'm still hood rich."

(Quick aside: Is Kelly misusing the phrase "hood rich"? Surely he means he's simultaneously rich and true to the hood, right? After all, according to urbandictionary.com, "hood rich" means a "person with extravagant luxuries that they clearly cannot afford, and lives in the hood or has a hood lifestyle.")

Meanwhile, also in "All of the Above," Sigel claims to pull "seven digits clean soon as I grace the stage" — odd, considering that his new album sold about 50,000 copies its first week and debuted at No. 37.

Equally preposterous is the title of Sigel's protégé and fellow Philadelphia rapper Freeway's new song, "Roc-A-Fella Billionaires," a collaboration with Jay-Z, who Forbes says earned $34 million in 2006 — more than any other rapper. This does make the song's chorus — which perplexingly downgrades their status to "Roc-A-Fella millionaires" — more believable. But that still doesn't explain how Freeway, whose latest album, Free at Last, sold 36,000 copies in its first week, managed to acquire "30 mil in the bank, 30 grand on the wrist and 20 mil in the Swiss."

There are probably other rappers claiming to be billionaires. Pharrell Williams, for one, has a clothing line called Billionaire Boys Club, but that sounds kind of pedophilic, so we're not going to talk about it. But we will use this interlude to remind everyone that having a billion dollars isn't the same as having a recoupable six-figure advance and a few nice cars. Billion has nine zeroes; it's the same as having $1,000 million.

Inflating your own worth, of course, is an American tradition; Donald Trump has made it into a career. But rappers seem to be among the worst offenders. Russell Simmons, for one, famously admitted to deceiving the public about sales of his Phat Farm clothing line. The New York Times reported that, in a 2004 civil deposition, he said, "It is how you develop an image for companies. So, in other words, you give out false statements to mislead the public so they will then increase in their mind the value of your company." Though Simmons had claimed the line sold $350 million in 2003, the actual figure was less than one-twentieth of that, or not even $20 million.

Much was made, meanwhile, about 50 Cent's stake in Energy Brands, after its brand Glaceau (which includes Vitamin Water) was sold to Coca-Cola last year for $4.1 billion. Though Fiddy — judged 2006's second-highest earning rap cat by Forbes — did nothing to quash rumors he was a 10 percent stakeholder in the company, which would make him a near-half-billionaire, the Energy Brands folks did. They called those rumors "erroneous."

Not surprisingly, there's even a label called Billionaire Records based out of Crockett, Texas. The label's name is likely something of a misnomer, because as of last week it had only a single friend: Tom.