Are you talking about fuel cells using either hydrogen or natural gas? If so, the technology is too new and limited in use for a meaningful price comparison. Both have been built many times but not in significant commercial production.
About a decade ago I tried to get into a pilot program for using a natural gas fuel cell as a stationary generator for reducing electric demand. One of the waste products of the fuel cell is heat. I had a swimming pool and was proposing a system to heat the pool. It never went anywhere and I wasn't willing to spend any significant money on my own.
Yes. For the reasons you mention, I don't know how one would estimate it, but obviously if by the time you got it into a car it was the equivalent of $20/gal gas...