I've just booked a flexible ticket for going out to the states for a month in January. Its going to cost me £350 plus $150 if I decide to change my return date. I just looked on lastminute.com and kelkoo.co.uk for the best flexible tickets. As regards one way flights, I believe you can only buy these when you are on particular kinds of visas, although I might be wrong about that.
Cheers
PW