blank 100GB BDXL disk costs $80
A 5-pack 100GB costs $255 here: http://www.digistor.com/Blu-ray-Recordable-Media/100GB-BD-R-Printable
I searched and couldn't find any 128GB blank disks. I'm sure when they're available for sale they will be even higher than the 100GB disks.
I'm sure the technology is reliable but it is definitely not affordable.
Also, there are no rewritable disks in BDXL format; so you can use each disk only once.