I was provided the $699 M4 Pro 4TB SSD upgrade by M4-SSD. It's quite expensive (especially compared to normal 4TB NVMe SSDs, which range from $200-400)
Depends what type of flash that's comparing. QLC is cheap, TLC a bit more expensive, MLC nearly unobtainable, and SLC insanely expensive unless you SLC-mod a QLC drive.