It'd be worth it just because defenders would be scared of anyone that cost that much, and Jovetic is good enough to capitalize on that (unlike the Horseballer) even if he isn't the fully finished article just yet.
We could afford it if Van Persie stayed, I think, but it would be somewhat risky. If Fiorentina really are as financially fucked as it seems, though, then replacing him with an actual player might be better for them in the end, even if it is just Marouane Chamakh.