Water heaters (both gas and electric) in the attic have become very
common here in the last 10-15 years. I personally think it is insane,
would never have one, and if I ever had a house with one in the attic,
would move it. The water here is very hard and after a few years a w/h
is likely to have a foot of what amounts to rock inside it. It must take
4 people to get one like that down out of the attic. As to the OP's
question, all you can do is get several prices from reputable companies
and compare-- prices vary greatly from one area to another. Larry