Billionaire Liu Yiqian of China has recently made what may be the largest-ever American Express purchase, using his Centurion Card, otherwise known as the "AmEx black card."

Liu, whose net worth Forbes lists as $1.4 billion, is a known long-time collector of big-ticket antiquities and art. Earlier in November, he was the winning bidder at a Christie's auction of painter Amedeo Modigliani's "Reclining Nude." The piece cost Liu a cool $170.4 million. That's more than the entire individual net worths of many of the world's richest people.

And, given that Liu's AmEx has no official spending limit attached to it -- the most well-known and coveted feature of the black card -- according to American Express spokeswoman Elizabeth Crosta:

In theory, it's possible to put a [$170 million purchase] on an American Express card. It is based on our relationship with that individual card member and these decisions are made on a case-by-case basis, based on our knowledge of their spending patterns.

And for that, Liu and his family may enjoy much more than a painting. They could ostensibly travel anywhere in the world, at any time, free of charge... for the rest of Liu's life. Explains editor-in-chief of ThePointsGuy.com, via KX News:

...if Liu converted his Membership Rewards points into one of a number of airline frequent flier programs, he and his and his family could travel anywhere, in style. He could fly 3,000 times between the U.S. and Europe in the ultra-deluxe first class suites offered by Singapore Airlines (estimated cost: $17,800 round trip), if he converted points to Singapore's program. Even if Liu wanted to use his AmEx points to pay for flights, a less efficient use of them, he could still redeem those points for hundreds of first class flights anywhere in the world.

Now you know just what it costs to fly "free" for life!