With all GTSs sold, I'd think the logical conclusion is that BMW priced it about right. We can't separate volume from price, as volume is an important (if not the most important) factor in deciding what a particular item will cost. I don't see it as competing against the GT3 RS, but merely augmenting what was already available in the field. It's just their take on what a track-focused M3 should be, and a nice anniversary edition which will appeal to someone who doesn't want the usual, obvious trackday toys. Burly V8 soundtrack (that also revs), competition-inspired paddleshifter, predictable (and driftable) handling...there's a lot to like that's different from the Porsche. R8 GT and MB Black Series are in the same vein, yet all are different enough to justify their prices to a select few. They're not overly ambitious. With only so few built, the market goals are quite different. We would not consider a sprinter to be "competing" against a marathon runner when he stops at the 100m mark.
A lot of this perceived competing and Top Trumps attitude is created by media hype. When the Scirocco R was introduced in one of the Brit mags, they somehow concluded this was VW's answer to one of the Caymans on no other information than near-identical hp numbers. To my knowledge, VW made no such claim and if they did they should have their heads checked.