Custom, or vanity tags, are pretty popular around the world. Even still, they usually fetch just a nominal sum by local government entities. But then there are folks like Afzal Khan. About a decade ago, he snagged a British license plate with the coveted "F1" designation. He's now decided that it's the right time to try to make a profit on his investment. 

He spent the equivalent of just over a half million dollars on the plate in 2008. Since then, he's displayed the license plate on a series of exotic cars from his collection.

While the princely sum earned some mockery, he saw an opportunity for future profit and is now hoping to cash in. The "F1" plate is now on sale for a staggering sum -- just shy of $21 million.

While it is unclear whether Khan will receive his asking price, it is obvious he sees great value in the plate's alphanumeric message. F1 is seen by some analysts as the most sought after custom plate in existence. The tag references the renowned Formula One racing series.

In the U.K., motorists can purchase their own tags, unlike other nations in which they belong to a government agency. This means investors like Khan can speculate on future value and make an early bid. That's exactly what he did with his "F1" plate, among others. But he's not alone.

A mogul in the United Arab Emirates answered that question with a 'yes' when presented the opportunity to buy they elusive tag. To be exact, Saeed Abdul Ghafour Khouri shelled out $14.3 million for the desired message. He obtained authorization to copy the tag, which he then displayed on his fleet of exotic cars.

Single-digit plates and culturally significant messages have gone for huge sums in countries around the world. Sometimes, though, a license plate is valued for its own intrinsic worth, not the personalized message it displays.

Among the costliest license plates sold in the U.S. was a rare example of an early 20th century Alaska tag. Travel by auto to the remote region was difficult, especially before the 49th state was admitted to the nation. So the 1921 plate that sold in 2000 -- one of only four known examples in the world -- went for $60,000.

There are plenty of examples of people spending enough on a tag to easily purchase whatever car they chose to place it on. For more examples, read this list of the 15 most expensive license plates in the world.