100 euros per 5×100 GB for 1000 years
M-Disc technology is full of promise for long-term preservation; I won't live for 1,000 years, but the data I burned may well last a few centuries at least!
In any case, the burning worked without a hitch, for a price not much higher than the price of classic Blu-rays of the same capacity - 100 euros for 5 x 100 GB discs, long gone are the days of CDs or floppy disks!
Photo, sound and video memories or even final documents, it's worth betting on the future (while continuing to make replications on other less perennial media, as a precaution).