The current market price is based on current supply and demand. Splitting water to create enough hydrogen for non-trivial fraction of the transportation sector would generate an enormous amount of oxygen. The price of oxygen would likely tank in that situation.

Except you can make oxygen for pretty cheap using oxygen concentrators. The technology is simple enough that home versions exist for patients with lung problems can lug one around at all times to have a feed of oxygen rich air. Oxygen is almost 21% of the air we breathe, it's trivial to capture. Hydrogen counts for only 0.000055%.