"Nainggolan is worth more than 15m," Rossoblu sporting director Nicola Salerno told Sport Italia. "Only President Massimo Cellino knows the precise figure.
"However, it is pointless talking about it right now as there are no negotiations taking place. There is time though.
"We are not interested in any player plus cash deals though. If someone wants Nainggolan then he will cost that much."