I don't know about prices, but the quickest way would be Eurostar to Paris, followed by the new cross-border TGV service from Paris to Figueres (there are two trains a day), which connects directly into a Renfe Altaris train to Barcelona.
From Barcelona there are several trains a day to Salou - it takes about an hour to 90 minutes depending on the class of train.
You can't do the whole journey in the same day (but you could get as far as Barcelona). The alternative is an overnight stop, or you could travel on the Renfe trenhotel "Joan Miro" from Paris to Barcelona.