top of the range BMW coupes either sell on the back of real/perceived sporting prowess, or on bling/luxury.
Successful BMW coupes have done better on the former than the latter; e.g.
E9 -racing CSLs assured its reputation;
E24-started out more show than go, but touring car wins (635 with M30 engine) plus M-car gave the model a sporty lift.
E31 -bling/luxury out and out
new 6er; no racing = no street cred?
BMW coupes have always been more expensive than any comparable saloons; M6 vs M5 isn't much different in percentage terms from what has gone before (although you can buy a VW golf on the difference...).
If the M6 were demonstrably much faster than the M5, it wouldn't be an issue; as it is, its only a bit lighter, some of the gadgets are a bit different, and thats it. Round a race track there won't be a big differential between them. Not £20k worth, anyway...
I think they might get away with +£10K over the M5 with this model, and that a genuinely much lighter and much faster version (say with 200kgs less, 600bhp and race wins under its belt) might command a £20K premium.
The V10 engine is nice enough, but lack of F1 wins, plus an upcoming F1 regulation change (back to V8s) take the shine off it big time.
To make matters worse, the older V8 engine can be tuned for the road (eg supercharged Alpina) to give the same power, but with a fatter torque curve. And it is lighter.
my two pence worth....
cheers