According to media reports teams have offered from 15-25m. So yeah, you're dead on. That seems his market value.
He's 27, has a poor injury record, and is a busy midfielder but doesn't really score or create much. Found a little role with Argentina but that just puts him in the Rodrigo De Paul category. He's an alright footballer who, if he stays fit, does a good job for a Villareal or a Lazio.
Talk of 50m is a pipe dream based on nothing more than some nice Ange comments and a couple of good pre-season games - we'd bite a teams hand straight off if they offered 40.
His big thing to say is that he plays with Messi and gives Messi many key passes when playing for Argentina .
Doesn't seem to understand that doesn't mean he's on a similar level as Messi and that a donkey might so a similar job of passing to Messi !