It doesn't depend.
If they think he could be our key player in the midfield (which is probably the thing, since they want him) then we should be ready to gamble and pay a couple of more millions. CSKA is playing it smart, if he wants Juve and Manchester City has offered 18 they want Juve to make a step and add more. Overpaying for some players is ok, if the story is developing this way.
You can hardly find some good players for that amount of money.
I really don't think they're looking at him as a "key" player in midfield. He is nowhere near proven enough to be considered as such.
If we are to believe everything we read, then we made an initial offer of about 13-13,5 million. Which CSKA deemed insufficient.
Apparently now we're up to 15 million, which is again insufficient.
At some point you just have to stop increasing your offer. Simple economic sense.
CSKA is obviously playing it smart, which is their right. But that doesn't really interest me.
And overpaying for players is never ok. Under no circumstances whatsoever.