Sorry that I didn't get back about this. The fact is that I don't know what it would cost. There are solar system suppliers here, who can probably give estimates, but it would also depend very heavily upon what sort of building you intend to outfit. For one thing, it would definitely be far cheaper to integrate the system during construction than to retrofit a 96 year-old house like mine.
Now that I've read Chi's thread through the link, it appears that I misunderstood the OP. I thought that he meant a hot water home heating system, not just a water heater. I still don't know what it would cost, but the type of building wouldn't make nearly as much difference as I indicated.