InDesign CS5 can be purchased as a stand-alone app. On Adobe's site, it is $699 (and
much less on eBay) where as the
CS5 Design Standard package (InDesign, Photoshop, Illustrator) runs $1,299.
However, talking to our people here, CS5 InDesign is running a little buggy right now. I would stick with an older version, which you can probably pick up relatively cheap on eBay (though I would stick with V3 or higher).