Here are all +20m eur signings in serie A in the last 5 years:
1. Zlatan Ibrahimovic, Juve to Inter........ ~25m eur
2. Alberto Gilardino, Parma to Milan.......... 25m eur
3. Amauri, Palermo to Juve.......... 22.8m eur
4. Alexandre Pato, Internazional to Milan............ 14m+8m = 22m eur
5. Ricardo Oliveira, Betis to Milan.......... 17m eur+Vogel = ~22m eur
6. Ronaldinho, Barcelona to Milan............. 21m eur
7. Patrick Vieira, Arsenal to Juve............ ~20m eur
If we buy Diego for 25m eur he will be the joint most expenssive player in serie A after the summer of 2002 when Milan bought Nesta for 30m eur.