I honestly believe there's a possibility of this transfer happening, but only if Bale has a terrible pre-season and start to the season.
The facts are - he's not appreciated at all by the fickle Madrid fans. He won't sell as many shirts as Madrid would like, being the world's most expensive player and all. And he doesn't get on well with Ronaldo - the fans God.
The "what ifs?" in this situation turn Bale into one of the biggest gambling chips in football history. Real know that they could get £100M from United who would quite easily spend that much for him, and that is extremely tempting for a heavily in debt Madrid over a player who had a nightmare last season, and who's marketability is depreciating all the time. If Madrid don't hit the ground running this season, after Barca cleaned up last season, there will be heavy pressure on Madrid to re-think how they can turn it around. Possibly to use the funds from a Bale transfer.
Bale to United is a possibility, but only a slight possibility. The player doesn't want to leave, but I'm not 100% convinced Madrid don't want to sell. They claim they don't, but that is natural in order to maintain his value.
The very fact that Benitez is forming the team around Bale, has to be seen as an attempt to try and get him ticking in order to justify Madrid's biggest player investment. The attention to Bale must be frustrating to Ronaldo, and the fans will get frustrated very quickly if doesn't work fast. The Whole Ramos situation is a smokescreen. United want Bale.