I will relent. I did my search on "liquid oxygen" but I now see that the source material is ambiguous---and I am running into dead ends trying to find alternative price data. So, if we construe the source data as gaseous oxygen, that would involve a proportionality factor of about 1000, leading to a price point nearer $0.50/gallon or $90/tonne. It gets more expensive in smaller quantities, so if you are not used to tonnes of the stuff, our numbers may be compatible. The point being, it is not very expensive for rocket propellant. (I am also relying on what I remember from DoD pricing specifications from the 1960s, in which it was nearly at the point of water.)
Jet engines are nice, but they can't get into space, so what is your point? That it is better to use an airplane to get part of the way there? Welcome to air launch. (I once worked for a design supervisor that was enamored of air launch, to the point of designing configurations to be launched from the topside of a 747. Some of these launch vehicles were as big as a 767 fuselage! The basic idea goes back to a science fiction story by Otto Willi Gail in 1928.)
I will relent. I did my search on "liquid oxygen" but I now see that the source material is ambiguous---and I am running into dead ends trying to find alternative price data. So, if we construe the source data as gaseous oxygen, that would involve a proportionality factor of about 1000, leading to a price point nearer $0.50/gallon or $90/tonne. It gets more expensive in smaller quantities, so if you are not used to tonnes of the stuff, our numbers may be compatible. The point being, it is not very expensive for rocket propellant. (I am also relying on what I remember from DoD pricing specifications from the 1960s, in which it was nearly at the point of water.)
Jet engines are nice, but they can't get into space, so what is your point? That it is better to use an airplane to get part of the way there? Welcome to air launch. (I once worked for a design supervisor that was enamored of air launch, to the point of designing configurations to be launched from the topside of a 747. Some of these launch vehicles were as big as a 767 fuselage! The basic idea goes back to a science fiction story by Otto Willi Gail in 1928.)