I was quoted $3500 up to $5000. A lot of the additional cost was for routing gas pipe to it. Apparently, they can't always use the existing gas line. The national code money pit. Always something. They're a great idea if you can stand the incremental expense.

I replaced my conventional 50g gas fired water heater with a similar unit for slightly under $2k.
It should last ten (10) to fifteen (15) yrs.