In 2001, Thuram cost 40mil and was considered a rightback at the time.
Defenders in general are lower value'd. And plenty of teams that value good defenders, get them from within the league. Nowadays good fullbacks are extremely rare. If they are remotely good at going forward, they cant defend like $#@!.
Meanwhile, when we look back at Maldini's days, we saw several fullbacks in serie a alone, that had technique and tricks one only associates with creative players these days, but beeing hard as balls in defence.
Madrid made a mistake. While both Danilo and Sandro are excellent players, i'd prefer Sandro.