Stefano,

I don't mean to discount your approach, but, if I had to solve the problem
as you present it, I would use disk quotas to control the amount of customer
storage and leave amanda to do her work of coordinating the backups. There
are many solid and well documented filesystems that support user and group
quotas as I'm sure you know.

I would then offer customers a choice of "Backup Service Levels" in two
categories "Recovery Service Plans" and "Archival Service Plans". I'll leave
the specifics to you or others, but the costs for such services (assuming
amanda :) would be calculated based on runspercycle, tapecycle, runtapes,
customer quota, cost of the tapes, tape drive depreciation, and tape
management.

I think such plans would be easier for a customer to understand and their
data is always being backed up.

Doug Douglass