The 21-year-old has been strongly linked with a move to the San Siro this summer, but thus far Fiorentina have repelled the interest.
"It will prove to be a real bargain if [Adriano] Galliani manages to sign Ljajic," Mihajlovic told La Gazzetta dello Sport.
"I know Adem really well. He has started to perform consistently under me. He has developed really well. He scores a lot of goals now and is a flexible and complete player.
"He is becoming a great player and I wouldn't let go of him if I were Fiorentina. He's worth making a financial sacrifice for. They did it before with Stevan Jovetic when they renewed his deal and could do the same with Adem."
Ljajic has also been linked with Atletico Madrid.