Ouch, that's pretty steep price for additional 8gb ram. How does it compare to other devices in the same category? I'm out of the loop when it comes to laptops, for desktops iirc you can get one 32gig die of ram for cheaper than that.
It's definitely more expensive than simply buying a DIMM, but keep in mind that is "unified" memory and is part of the SoC, and is likely much faster than any old stick of RAM you'd normally put in a laptop. That, plus the Apple Premium.