Your Pirlo point raises an interesting question - did Pirlo have a positive Return on Investment?
As of this time last year, his jersey was the #1 seller: https://www.mlssoccer.com/post/2016/07/15/these-are-top-selling-mls-jerseys-mlsstore-com-so-far-year
Just a single data point in a complicated analysis, but signs point to him as being one of the most marketable signings in the league.