About the same cost as gasoline now on a per mile basis, and expected to decline as more cars are on the road
Type: Car
Short Answer:

Automakers include three years of hydrogen fuel with the sale or lease of a vehicle

Long Answer:

Hydrogen fuel prices range from $12.85 to more than $16 per kilogram (kg), but the most common price is $13.99 per kg (equivalent on a price per energy basis to $5.60 per gallon of gasoline), which translates to an operating cost of $0.21 per mile. Automakers are including three years of hydrogen fuel with their initial sales and lease offerings, which will shield early market adopters from this initially high fuel price.

While future price is uncertain, NREL estimates that hydrogen fuel prices may fall to the $10 to $8 per kg range in the 2020 to 2025 period. A kilogram of hydrogen has about the same energy content as a gallon of gasoline. FCEVs are about twice as efficient as gasoline-powered vehicles: an FCEV travels about twice as far as a conventional vehicle given the same amount of fuel energy. At $3.50 per gallon gasoline, a conventional vehicle costs about $0.13 per mile to operate, while an FCEV using $8 per kg hydrogen fuel would cost about $0.12 per mile.

Reference: http://www.energy.ca.gov/2015publications/CEC-600-2015-016/CEC-600-2015-016.pdf (page 7)