Honestly the price differential is very small between 4 and 6 layers now-a-days. In mass pro that is, they will still charge a solid premium for 6 layer at many quick turn / small batch PCB manufacturers.
The biggest savings comes from avoiding blind and buried vias which are still expensive to use but often required for dense BGAs. You can also usually get away with a larger minimum feature size (trace width) with a QFP which can also be a nice cost savings in the PCB construction.
Comments
Honestly the price differential is very small between 4 and 6 layers now-a-days. In mass pro that is, they will still charge a solid premium for 6 layer at many quick turn / small batch PCB manufacturers.
The biggest savings comes from avoiding blind and buried vias which are still expensive to use but often required for dense BGAs. You can also usually get away with a larger minimum feature size (trace width) with a QFP which can also be a nice cost savings in the PCB construction.