It's all relative. I think Jojo is a better player than Diego, but the latter we paid 25 M for. Cavani isn't worth 60 M either. He's amazing but he's not 3 times better than a 20 M player like Higuain. The point is, a players value doesnt reflect so much how good he is as a player as much as how much of he's seen good form in the last year or two, and most importantly, how much his club value him
For example, a player like Quag is worth less to us than he would if he playing for Palermo. So how much Jovetic is worth is not dependant on his abilities per se. Its mostly about his form and this year, hes worth less than last year because of injury. So I'd say he's probably worth around 15 to 20 M now, but if we can get him, and he plays regularly for a season or two, his value could easily double.
That's why this is the best time to buy him, his value is as low as it could get right now.