After I've booked economy class tickets on United, they frequently offer me the chance to buy up to business/first class. For a 3 hour domestic flight they usually want $300 for the round-trip. Which seems kind of steep, although if you were planning on buying "economy plus" (+$100) and checking a bag (+$50) then the incremental cost to get to first is only $150. Still more than I want to spend on a 3 hour flight. I think at +4 hours, the $300 upgrade would be worth it. But of course they want more than $300 for those longer flights.