I would have thought that the truely rich would charter a personal jet, but for the rest of us that have to slum it in First/Business:
There are the very top earners who either have made their pile and now are leading a lifestyle they have become accustomed to.
Then those whose business are paying at cut down negotiated rates.
Those who have hunted high and low and booked months in advance on a very good deal.
Or those using their air miles.
I've not much personal experience of 1st but once did ORD->MAN on AA and there were only 3 of us in the cabin when business was quite full. So obviously the price for most people wasn't attractive enough (the company was paying for me).
Another time I did LHR->SFO and all 14 seats were taken both outbound and return, and I'd say about 1/2 of those were business people (the laptop gives it away), or individuals and couples just travelling and making it look like they did this regularly. The LHR->SFO return in 1st at that time was about £10,500 on the web site. A staggering amount of money, but there is obviously a market for it.
Probably the most interesting point on 'how much' came from the Concorde days. During the 80s, after privatisation IIR, the marketing team asked the business passengers how much they thought the ticket cost, or, more importantly how much they thought it was worth. As most of the bookings were done by the travel department the execs never actually new the cost of the fare. It turned out that BA were 'undercharging' and so simply raised the fare and increased the profits...