Portfolio.com figured out what exactly what Stairway to Heaven by Led Zeppelin is worth. Here is an excerpt from the article.

Zeppelin hasn’t licensed “Stairway” for movies or commercials. But songwriters Jimmy Page and Robert Plant and Warner/Chappell, the song’s publisher, make money off royalties from record sales, radio plays, and live performances. Zeppelin has played “Stairway” at every gig since 1971, yielding about $150,000 in royalties. Everyone from Frank Zappa to the London Philharmonic has also performed it, and let’s not forget the hundreds of thousands of proms, weddings, and bar mitzvahs where it’s been played. (D.J.’s and venues pay a small annual fee for the right to play it.) Estimated gain: $400,000. Royalties from album and DVD sales total about $8.6 million. Plus “Stairway” has been played on the radio an estimated 2,985,000 times (equal to more than 45 years of uninterrupted airtime), netting nearly $2 million. It’s also thought to be the bestselling piece of sheet music in rock history, with royalties of $1 million.
ESTIMATED TOTAL: $12 million

Now don't you wish you could write that one song masterpiece?