The Bundesliga champions have stressed that they will only sell the Japan star if an offer satisfies them as the club's chief drew a transfer comparison to Manuel Neuer
The Japan international is keen to leave BVB ahead of the 2012-13 campaign, and has set his sights on a move to the Premier League.
Manchester United are believed to be the frontrunners to secure the playmaker's signature, but the German giants will only sell Kagawa at the right price.
"It's pretty obvious what Shinji's qualities are, how old he is and how valuable he is for this team. Then I look at the money Bayern paid for Manuel Neuer. He had one year left on his contract, too. You can derive a sum from that," Watzke stated to Sport Bild.
Bayern paid Schalke an initial transfer of €18 million for Neuer, with the fee potentially rising to €25m.
The 23-year-old Kagawa played a key role in Dortmund's title-winning campaign as he netted 13 goals in 31 Bundesliga appearances.