The real question is why are you looking?
If you want to add on internal flights to an international booking, then you are almost always better booking a complete trip
For example Manchester-Chicago-Dallas-Chicago-Manchester will be MUCH cheaper than
Manchester-Chicago-Manchester and Chicago-Dallas-Chicago
ALSO (and no small point this) a booking including a transatlantic xector will give you a 2 bag baggage allowance (with no 20Kg weight limit per bag) on ALL sectors in the booking
If you book seperately, you will possibly be hit by a 20kg per person limit for the internal flights
To answer you question though, if you are looking for CHEAPEST flights, try www.southwest.com