We took the Eurostar last month. One way tix are very expensive, so we bought r/t tix and threw the return half away. About $60/person in total.

You have to get to Waterloo Station, and you wind up in Paris at Gare du Nord. Transportation to/from either location is easily obtained and relatively inexpensive. You have to figure in transportation to/from LHR and CDG in your calculations. It's probably close to a wash, with maybe the plane fares etc coming out a bit ahead. The luggage may be a problem with bMI - not sure, but it often is with Ryanair and Easyjet - check first.

The train ride is actually quite nice and the scenery is enjoyable. I wouldn't be surprised no mater which option you chose.