using random dates (out 6th, back 14th) and a flight broker (majortravel, but they're not necessarily the cheapest), United come first, then Virgin, then American; though all are around the �450.00 mark.
Try http://www.kayak.co.uk/
to give you some comparisons. Then try the individual airlines websites to see if they are cheaper than on kayak. Remember a return will be cheaper than 2 singles.