mmm yea, very expensive cars indeed. buuuuut, i don't believe the most expensive... consider the 1962 Ferrari 250 GTO, also known as "pure sex". Enzo only produced 36 of them and are still often referred to as the "greatest sports car ever made" the last one that changed hands did so in 1991 at auction. the final bid, by a japanese collector, was $15 million.
the price of them is estimated to fluctuate currently between 12 and 14 million, but since none have been sold recently, no one's possitive of the exact worth. it all depends on the particular one of the 36 your looking at, and the racing heritage behind it. (i.e. how many races it won, who drove it, etc..)
cool article on the bugattis though. i'd also check up on the Dousenburg (sp?), i believe those go for quite a few million too.