Why is that Canon 1DX and Nikon D4s cameras are very expensive?

What are the specialty of those cameras? Isn't Canon 5D Mark III and Nikon D810 produce better pictures as those cameras? It seems to me that 5D Mark III and D810 offer better value for money than those insanely expensive cameras.

It's a combination of build quality, the ability to track focus at a higher frame rate for sports, and the fact that it's designed for the professional market adds on a premium too. It always seems that the pro gear gets a premium added on because the pros that use it can justify the costs since they actually make money with their gear.
Fact is that the 1DX and the D4S do something that none of the cheaper cameras can do, and that is focus as fast and crank out as many images in one second as they do. The new 7D Mark II comes in close since it was just announced a few days ago.

First, understand this: "It isn't the camera its the photographer."

The 1DX and D4S are built to a higher standard than the 5D and D810 to withstand the use and sometimes abuse that occurs when a professional photographer uses them for several hours a day every day under any and all conditions imaginable.

At http://www.imaging-resource.com you can do side by side comparisons of the 5D and 1DX and the D810 and D4S.

They are the flagships of their respective companies. It could be argued that the 5D3 and the 800/810 have better IQ, but for many pro photographers, the holy grail is HANDLING SPEED, and that's where the two cameras you mention come into their own. I've handled them both, and they are incredibly fast and responsive, way more than the higher-megapixel models.

Why is a BMW series 7 dearer than a series 3!

Larger sensor.

Better sensors, more range with the settings, less unnecessary point-and-shoot features.

It's a combination of build quality, the ability to track focus at a higher frame rate for sports, and the fact that it's designed for the professional market adds on a premium too. It always seems that the pro gear gets a premium added on because the pros that use it can justify the costs since they actually make money with their gear.
Fact is that the 1DX and the D4S do something that none of the cheaper cameras can do, and that is focus as fast and crank out as many images in one second as they do. The new 7D Mark II comes in close since it was just announced a few days ago.