It's all a question of who's value you're speaking about. United value him at about 12 or so million, since they have three LBs and don't NEED Baines like they need a CM. Everton, on the other hand, value him at something like 20-25 million, since their entire team is built around him. In terms of real market value, he's probably worth something in between the two valuations.
He's a big enough upgrade to make it worth it for the right price, I'd say.
You don't trust any British player that comes from Villa? The vast majority of British players who have left Villa, particularly recently, have done good work for their latest teams. It just so happened that Downing was a flash in the pan and Young turned out to be a bad small fish in a big pond (as I suspected), and those two were the most high-profile departures. And it's not like there's something endemic in Villa youngsters that makes them only good at Villa, it's just those two were always candidates for failing outside of Villa because of their individual attributes. ****, I predicted both could've been a bit dodgy, though admittedly not to these levels.