Barca paid just under £30m for Alves at the start of last season so I'm sure they'll bid a similar amount for Mascherano. I can't see Benitez turning down an offer close to £30m.
At the time Alves cost closer to £22m (even at today's rate it's only £26m (£4-5m of that is add-ons too)) and iinm that's the only time they've spent over £20m on a player.
And i can see us turning down an offer close to £30m for Masch.