Depends where Villa are come January. If they are in mid table safety then they will probably let him go for around £7-8 million. If they are flirting with Europe or relegation then we'll need to pay more to get him, as he's a quality player and Villa know the rewards from Europe and particularly penalties from relegation could far outweigh the cash they would lose if he runs his contract down.
I think he is a good contingency if United do not get their first choice. United should always go for 2 at a time. That will discourage the selling club from asking ridiculous prices . They will know that United will walk away and they may be left with a disgruntled player.
Pay way over the odds and when everyone moans chuck each premier league team a 5er with a note telling them to treat themselves.