Had a quick look (sorry not getting much time) From Barcelona it looks like getting to London is easier than Paris, Orbitz.com have a 1 way Barca-London with British Airways for $111pp on the 30th May, a little more for 29th May at $139pp
From London you can fly to CDG for $110pp incl for days I checked 3rd & 4th June
Then it gets expensive getting back to US. Only way I could see to get the price down was multi stops, Paris to NYC via Dublin $556 then NYC to Houston $164, a long way around but far better than direct.
The direct route from Paris to Houston is silly money $3,406
I just tried Orbitz for ease as its in $US for you. If its something you really want to do post exact dates, and route you want and I will search multiple sites but that gives you an idea of costs.