Have you checked out the rtn fares - they tend to be cheaper and you can just ditch the rtn section.

Also check out o/w from Rome to Brussels with Virgin Express (a low cost carrier) - might get something for about $70 and onward to Paris via high speed Thalys train (1 hr 30 minutes)($30) - should be able to do it all for about $100.

Or via Stansted - Rome to Stansted with Go
( $70 ?) connect with buzz to CDG for about $50 - a bit time consuming but cheaper.

No miles or anything but Go are fine (better than parent company BA IMHO)