One can argue the M3 uses better materials. Although having been in both and was seriously considering buying the M3, the difference is not 2x as great. If you’re a performance enthusiast I can see the cross shop.
The true reason why the M3 is a lot more expensive than the Mustang GT is BMW’s inefficient methods of sourcing materials, and more expensive labor costs. Heck both cars even ride similar.
From an end user perspective the M3 appears to be better built, but from a supply stand point the cost difference of the M3 materials and a Mustang GT is neglibible. BMW just has a lot more profit going into their car, whereas Ford is making their money off of volume.