You have to check all of them. For example, Travelocity has what they call Travelocity Value Fares (for air travel) and Travelocity Value Rates (for hotels). You can only get those prices if you book through them.

For air travel, you should check the carriers' sites as well. United Airlines' Internet Special Fares (not to be confused with E-Fares) typically reflect a 5% discount off the lowest published airfare.