How can I improve the cold floor in my ground-floor unit?
On the cheap end, I'd suggest laying down 2" XPS (The pink stuff), being sure to tape all the seams, and then laminate flooring on top of that. If you have heavy furniture, you may need to put plywood down prior to the laminate. If you can forgo the laminate, shag carpeting would probably offer more insulation on the feet. 

On the 10k side, you may be able to get a heated floor as Asaf suggests, but that'd depend on all sorts of variables. The may electrrical heating systems you can tile on (typically for bathrooms) as well as hydronic systems that would require another layer of concrete on top of. 

OH, and of course, the REALLY cheap option: invest in some really nice slippers. ;)