measuring disk usage of records

We've had a chance to talk to some of our competitors who have recently gone
out of business. One of the major failings that contributed to their
collapse was not accurately understanding how much disk space and bandwidth
they were using up. Therefore, they often lost money unexpectedly which, as
most of you know, is not sustainable.

Therefore, we'd like to avoid falling into the same trap and get a good
understanding of how much storage space our customer's data is actually
using.

Here's the problem... A customer's data may be scattered across multiple
tables. Some customers have X,000 records in table A, some have Y,000
records in table B and etc. There's no way it can be as simple as saying
"database takes X GB for Y customers so usage is X/Y"

Does anyone have any suggestions that can help me to get access to this
information?

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