I can't make an assertion that my example is the single most expensive plane crash in history, but it was at minimum incredibly expensive.

The Valkyrie bomber was designed to fly at Mach 3 at over 70,000 feet to strike targets in the Soviet Union. The fact that it could achieve those feats starting in 1964 was astonishing. But, those same figures were its downfall. Designed as a high-altitude penetrator, it was supposed to fly higher than anything that could bring it down. However, developments in anti-aircraft missiles meant that the Valkyrie was quite within range from the ground. And when you tried to go in low--like the B-52 was able to do--the tradeoffs necessary to achieve those lofty figures quoted above were just too drastic. The Valkyrie was barely faster than a B-52 on the deck, and its fuel economy left a good deal to be desired. Also, the bomb bay was limited in relation to the Stratofortress. The Valkyrie was an engineering marvel, but it was made obsolete before it was even built by the engineering marvel of the anti-aircraft missile.

There was just enough momentum in the program to get two examples built. The first one had some problems in its construction, as there were some new processes being used. The second example righted many of these problems. Then there was a request for a photoshoot. Sadly, one of the pilots flying an F-104 fighter in formation with the larger bomber got too close, and the wings of the two planes made contact. The F-104 pilot and one of the two pilots of the Valkyrie were killed. That happened in 1966, and the other plane was retired in 1969. That is when it was flown to Dayton to become a museum exhibit--one I visited as a child.

The cost for the Valkyrie bomber project was $1.5 billion in 1950's and 1960's dollars. With two examples built, that's $750 million per plane. So, with that logic, the plane crash cost $750 million, plus the relatively negligible cost of the F-104 (under $2 million). Adjusting for inflation would only increase the dollar amounts.

The remaining Valkyrie is still located at the National Museum of the US Air Force near Dayton, Ohio. It's located in the R&D hangar. Admission is free, but you have to sign up early to get a seat on the bus to the R&D/Presidential hangar. For famous and rare warbirds, it's hard to beat this museum.