Not sure where you are flying. In Europe, there usually is no such option. Most of the time, all you can do is pay an additional €30 or so and get a seat with more legroom. And that's it.
That’s true if you’re flying on budget airlines like Ryanair or Easyjet, but nearly all the legacy carriers offer a business class “cabin” where the seats are usually the same as in economy but middle seats are kept free. That’s a 33% reduction in density, not 50%, but it’s not far off.
Those business class seats on "legacy carriers" are not available for double the price. Try some flight searches and compare direct one way flights between two cities in Europe. First of all, you will have a hard time finding a direct flight by Lufthansa for example. Secondly, a Lufthansa business flight will usually be over 5x the price of a Ryan air flight.
Oh yeah, I doubt you’ll find business class tickets that are anywhere near as cheap as twice the price of the average Ryanair flight, but as someone who is in the lucky situation to be able to avoid airlines that I find actively unpleasant, the price hop from economy to business needn’t be that large. It depends on the airline, route and dates of course, but I’ve seen (and bought) short-haul business class tickets that were less than 150% the price of economy.
Exactly, I am(or well, have been) flying very regularly and I've never been on a plane with a business class, much less first class. The best upgrade is an extra legroom seat.
