Are you asking about a medical treatment – meaning like, a lot of different medications?
I answered a similar question about the most expensive medicine. I did a bit of research to find out if there are any conditions requiring multiple medications that would end up being more expensive than the medicine Soliris, which costs over €300,000 a year. I couldn’t find any, so I guess I’d say that this is the most expensive treatment, even though its only one drug!
One treatment that i know is very expensive is given to underwater divers that rise from the deep too quickly. The nitrogen gas in their blood can form dangerous bubbles in their tissues like brain, heart, muscles and lungs. This is a very painful and dangerous condition called “the bends” (there is a great radiohead album named after this by the way!!) that can be fatal.
The treatment involves putting the patient into a very large barometric chamber (a sealed metal room) that recreates the high pressure of the deep water. This makes the nitrogen bubbles re-dissolve in to the blood and water in our tissues. Then, you lower the pressure very slowly like what would happen under a normal diving ascent from depth, and you can cure the person.
There are not a lot of these chambers around, and they are very costly to build. I guess that if you calculated how much each treatment were to cost it would be a lot of money!