Also known as the A2, this supersonic airplane should be able to fly Mach 5 at a reasonable price:

Long-Term Advanced Propulsion Concepts and Technologies

From the article:

Analysis of the Development, Production and Operating costs suggests that the average ticket price would be comparable to an existing Business class ticket. Therefore in principle the A2 vehicle could capture all of the current business and first class traffic due to the greatly reduced journey time of 4.6 hours compared to the current 22 hours. This estimate assumes hydrogen fuel derived from water electrolysis whereas the ticket price would roughly halve if the hydrogen is produced by steam reforming. Also the analysis assumes no hydrogen subsidy, however in reality it is likely that the first generation of hydrogen fuelled aircraft would be subsidised to promote the switch to a more environmentally friendly fuel.

See also: MACH-5 A2: Fly Sydney to Brussels in 4hrs – Emissions Free!

[See #25]