I installed a Rinnai two years ago. At the time I was able to get it on amazon for around $1300 including the special vent pipes it needed. This was the 98i series, so enough to run 4 showers. The very first thing to do is to call your natural gas company and ask them to come check your meter and gas pipes. In my case, the original tank heater was in the garage and I put the new tankless in a closet that I built for it in one of the bathrooms, so I had to add 30 feet of pipe. Pipe sizing is important. The gas company representative will come to your house, look at your meter, look at the tankless unit that you want to install and tell you exactly what size piping you need, if it's different from the original pipe size. This cost nothing to have checked out. In my area, you need to have a permit to do *any* kind of water heater replacement at all, let alone, relocating it, going tankless, changing pipes, etc. I ended up doing all the work myself, with the help of a family member who is a plumber. I mounted the unit on the inside of an outside wall and cut holes in that wall for the vent pipe, the condensate drain, etc. It took a whole weekend to do the plumbing and get it all set up.
I've had it about two years now and have had no problems at all, the water gets hot quickly and stays a nice constant temperature, regardless of how long I shower for. I've even had 3 people take showers at the same time, with no issues at all. One time I did have one small issue.. It got cold enough outside that the condensate drain pipe started to freeze and the unit shut down with an error code. I looked up the code in the owner's manual and I found the frozen pipe outside. It's a copper pipe, so I applied some heat with a propane torch to thaw it out, and it was all back up and running nicely. It doesn't get that cold where I am very often, so I don't feel a long term solution is needed for this. I can always temporarily disconnect that drain pipe and put a 5-gallon bucket under the drain for a day if needed.
