COM/NET/ORG are not going anywhere. These are the Big 3 and have grown organically with the internet.
They have widespread usage, awareness, and credibility.
Many brands have far more value in .COM which does not translate to NET/ORG.
.ORG is better for keywords that fit as well. For instance a term like Forklifts is going to be a way better fit in .NET than .ORG, and therefore be worth a lot more in .NET.
There is no magic formula for pricing. When it comes to pricing it depends on the term.
Brad