The DELL1 server is then backed up using Microsoft Data protection Manager (DPM). Changes get copied 3 times daily to it. This also goes for our production databases. The advantage is that we can go back about 1 week on disk without having to dig out a tape. We are currently using 10% of the capacity (I just upgraded it from its former 1GB size which was at the limit. The numbers don't quite add up yet but we are still building daily backups. But 10% is ballpark.)

Once a week on Saturday we also do a tape backup. That is at 50-55% capacity and is a function of tape size and tape drive capacity.

Tapes aside, reserving another 20% for any expansion I need for production work, and another 20% for overhead I have room 50% room currently free on the DPM system….But…..
That disk space is NOT available to General Windows use. The best thing then is to acquire (or reuse if possible from Monroe) a separate server to hold the scanned documents, and then back them up to the DPM. I’d set up a separate protection group for them so that the tape backups stay separate.

Also there will need to be an offsite storage arrangement for the tapes.

Cost: reusing a server, it will only be the cost for the required tapes, and the required DPM agent license for the server.
What I don’t consider here is retrieval times or available storage. If an available server is out of disk space, or too slow then you need to add in the cost of a new one (and figure $3000-$4000 for the hardware and OS).

In simplistic terms, a server will always out-perform a desktop where multiple access is required in both speed and reliability hence it's bigger price tag.

What you should perhaps concentrate more on is the fact that it is very rare to have to access the old data.