This is only my experience on long flights. In 2007 I had a $600 coach seat from NYC to Ireland (may have been a connection I do not recall). The first-class seats were in the upper $4k range.
My wife and I fly from the northern east coast of the US to the southern east coast once a year, the tickets are roughly $650 total for coach (including $50/person for the "Comfort +" seats). First class has never risen past ~$950.
Part of it is that there's a huge delta between international economy and international business/first while the difference in domestic is mostly wider seats, free drinks, and maybe a meal. Hey, I'll take the domestic upgrade but it's not as big a deal as on a long international flight.
This is only my experience on long flights. In 2007 I had a $600 coach seat from NYC to Ireland (may have been a connection I do not recall). The first-class seats were in the upper $4k range.
My wife and I fly from the northern east coast of the US to the southern east coast once a year, the tickets are roughly $650 total for coach (including $50/person for the "Comfort +" seats). First class has never risen past ~$950.