Wow. Things are getting a bit harsh here. I'll try to be nice.
As someone who has owned an E92 M3 and now has a 991, I can honestly say there is a justifiable reason for the price difference. In Canada, a well equipped M3 will run around $94,000 after tax, and a 991S will run around $130,000 after tax.
I've spent a huge amount of time on the track with the 991S, and I'll begin by saying that the Porsche has brakes and then some. The OEM M3 brakes just don't have the heat capacity (I spent a fair amount moving up to a set of Alcons and track pads in the M3). Right here, the Porsche justifies $10K in price difference - I ran my 991S with OEM brakes/pads for 14 track days with meat still left on the pads. I glassed my OEM M3 pads after one track day. My Alcons solved the problem on the M3, but they weren't cheap.
The interior of the 991S is at least another $10K leap in quality. Partly due to the insanely stiff chassis and quality of the interior work, the car just doesn't make any noise. I couldn't say the same about the M3 after a year. The leather used in the cabin is extremely good quality, and the seats (i've got the sports seats plus) make the M3 seats feel like benches rather than buckets. If you've spent any amount of time in the 991 cabin, the differences become dead obvious.
And finally, the suspension, engine and transmission combination is worth at least another $10K if not more in the 991S. I've run the stock PZero on my 991 for over 14 track days, and the wear has been absolutely fantastic. The car is very easy going on its tires. The other thing is the gas mileage. Driving down from Toronto to the Glen, I used about 3/4 of a tank, even though I was often cruising at over 100mph through NY. My buddies with M3s used practicality double the amount of gas, and our cars put out pretty much the same power. And the car is phenomenally faster than the M3 (in the right hands - not necessarily mine
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I'm sure some of you will call me out saying i'm a biased 911 owner, but I can say the same about you M3 owners (and don't forget I was one). Looking at the amount I spent to get my M3 track worthy makes the price difference between the 911 and the M3 marginal at best. Frankly speaking, if you don't push the car to it's limits (i.e. track regularly), the difference between the two cars is probably not justifiable. But if you use the 911 (IMHO) the way it should be, you'll see that it definitely is worth it and then some.
That being said, the M3 is an amazing car. It is well built, good looking, fast and easy to drive. The E92 M3 can probably do 8/10ths of what the 991S can, but frankly, the remaining 2/10ths is worth the $30-$40K difference to me. Oh, and don't compare the Mustang GT to the M3, compare it to the Boss 302 Laguna Seca. Yeah, the interior sucks, but man that car is a thrill and a half. Pretty much the same performance as the M3, but 2/3 the price.