From my own experience (from MidWest, live in SF now), $500 is pretty average for a cross country. I can get most places I want to go (any major US City not completely across country) for about $150-$300 though and these are all round trips. It's just when I go from SF <-> NYC/Boston/DC that I get the $500-$700 range. Or if I just book at horrible times or over holidays.