I use B2 for my restic Backup. Currently i have about 100GB stored there and it says i have about 100,000 files stored.

With the adjusted calculation of the blog you posted for 10TB of 50KB files, adjusted to the mean file size of my restic repository 10 TB of 1MB files:

Transaction fees: 10,000,000 / 10,000 x $0.004 = $4

My Laptop has a mean file size of about 16 MB per file on the local restic repository.