Sounds like you might need to do a risk analysis.
What are you trying to accomplish with this backup. You said that the systems have their "...own UPS and backup". Why do you need another backup?
Certainly an external USB drive is the cheapest way to go. But that doesn't protect you much more than a mirrored drive. That also doesn't provide protection against corrupted data. Once the corrupted data has been backed up you've overwritten your only copy of good data.
But what I would consider is taking an image of the system when it is built. Then you can always restore the system to its starting point. You will then only have to back up the database on a regualr basis. You would also have to create a new image of the machine any time the software was upgraded.
But you still have the problem of backing up an active SQL database. You'll need expensive ($1,000) software to control this, or you can do it manually if you don't need the system 24x7.
If you want to be able to back up all the systems to a central location, then you might want to consider a replication server at each site. Then you can backup the replication server over the wire, and not have to take down the active servers.
Of course if you have large databases then backing up over the wire won't be fast enough to keep up. Not to mentino that backing up over the wire is much less reliable than backing up locally to tape.
