Aye, he's not worth more at the moment. When you buy someone like Jovetic you pay a lot of money based on what he'll be worth in the future though, and he's got enough talent to be one of the best attacking players in Europe one day. There's no way Fiorentina will sell him for that price in a January transfer window unless they have the worst sporting director in the world (they don't).
The rumour is baseless, anyhow. Serbian newspapers picked up a dubious English weblink and in return carried the story to Florence and it looks like it has spread from there. There's nothing in it.