Hi David,
The figure of 80GB compressed is just an estimation based on assumption that you will get a 2:1 compression ratio. For some reason all the tape manufacturers do this (though I think some use 3:1 as the base), yet this has little relevance to the real world.
In the real world where you and I live, different types of data can be compressed at different amounts, for instance if you took a large text file or .bmp and compressed it with winzip, you would find that it compressed a hell of a lot. In fact a 5MB text file can be compressed to about 400KB!
The compression works by removing all the data which is empty or repeated, and replacing it with a pointer to allow the software to restore it afterwards when required.
Files like .jpeg's, .exe's etc don't generally have much useless data, so don't really compress at all.
So depending on the mix of compressible and uncompressible data on your server, you'll see a lot of difference with the ratio you actually see. From my own experience, and comments from others, I think you can realistically expect to get a compression ratio of about 1.2:1. Anything more than that and you're very lucky.