In short, millions!
It's not just a CATIII ILS (Around £400k) you're looking at but the lighting too. Extra circuits for TDZ, Supplementary lights, upgraded Approach, stop bars, Centreline lights and Runway Edge if you've not already got them, possibly a new AGL controller. Plus signage for CATIII Holds, IRVR, training of staff etc.
If it was cheap, every airport would be CAT III.
Does that answer your question?