Having worked for several "High net worth individuals" for many years now I can tell you the economics of this will work. There are no shortage of individuals willing to pony up $120 million for a Supersonic biz jet, it's as much a matter of prestige as it is about time savings. A friend of mine who flies for a wealthy Saudi family recently told me that the principal had a just bought a new 777 for his wife to use on shopping trips because the 767 they had was too small!
As far as fuel economy goes; it just gets better with each new generation of jets, the G650 I currently fly burns the same fuel at M.87 as the G550 burned at M.84, although we almost always cruise at M.90, in fact in the past 2 years I've only made a handful of trips at less than Mach.9 and at that speed I still get better fuel burn than I did in the Gulfstream III at Mach.80