That is common practice, not just in modern art. Unknown soccer (and probably other sports too) players transfers are also used to launder money, or simply to move big amounts of money in ways that could raise questions if done through normal operations. Who defines a player's value? All it needs is building a fake career, have him score some nice goals, arrange some set up friendly matches and shoot videos that will make him look like a champ. Then nobody would complain when his transfer will be reported as amount X while in practice X/10 is the real transfer cost and the rest is money being moved under the table from A to B. That practice is harder to do in top leagues, but easier to conceal in minor ones, and still profitable.