Ever since allegations of bribery and corruption in Wal-Mart's international operations emerged two years ago, the world's biggest retailer has spent more than $400 million in legal fees and compliance costs. That makes it one of the most expensive probes in US history. It's not over yet: Walmart is conducting its own investigation, cooperating with a federal probe, and paying legal fees for dozens of executives. In February, Walmart estimated that its bribery probe and compliance costs would total $200 million to $240 million for the year.