You can get similar deals on A8's, even S-classes, from time to time. The bigger and more expensive the piece of metal, the harder they have to subsidise them to get them on the roads.
I nearly bagged one of these 640d's a few months back on a similar rate. But I walked away, and I'm much happier to be getting into a 330d. Why ?
- better ride and steering feel;
- far more interior room especially vs exterior size;
- better visibility / parkability;
- less vulgar/likely to alienate friends who are struggling at the moment, especially when on school run / social trips;
- similar real world performance;
- similar refinement (road/wind noise in the 6 series is poor for the class of car).
I honestly can't think of one trip I've taken in the last 12 months where I'd rather be in a 640d than a 330d. And that being the case, it doesn't matter what the list price of the 2 cars is, right ?
If you were going to spec up your 330d with pro nav and other goodies that come as standard on the 640d, maybe the 640d would actually work out cheaper and then maybe you should consider it, but at £100 pcm more ? Definitely not.