For me: 63k images, around half raw (mostly 12Mpixel D300 images):
- Images take 427Gbyte
- Catalog takes 1.25Gbyte
- Previews take 11Gbyte
The size of the previews will depend on whether you've looked at everything. I mean, when I moved everything to a new drive last year, I didn't copy the previews. That means there will be previews only for things I've looked at since then.
You don't need to back up the previews in the previews folder called something like "Lightroom 4 Catalog Previews.lrdata" - you can even delete them if you like, but LR recreates them next time you look at the relevent image. Obviously, looking at an image in Library module is slower if the preview isn't there.
Whatever, the space taken by the catalog and previews is pretty small compared to the space taken by the image files.