I just finished installing a Rinnai C53i as a DIY project. I chose Rinnai because I only have 2x8 joists in my small house, and their zero clearance vent allowed the vent system to run through my limited joist space.
When I was shopping around, the best price I could find was on eBay, where the unit was $600 plus $50 for shipping. It is however a commercial unit, which lacks a bypass between the hot and cold side (My understanding is this is required in some areas as an anti-scald feature). Also, the temperature range of the output is adjustable between 120-180 degrees F, whereas the residential unit has a "colder" range, with a lower limit of 100.
I bought a valve/pressure relief kit and termination kit locally which came to around $200. Plumbing parts for the gas and water lines came to about another $100. I'm also participating in the Canadian ecoEnergy program, so the $300 incentive will make the project quite cost effective. With the incentive, it's cheaper than a power vent tank system.
I'm happy with the system save one issue. It takes significantly longer for the hot water to get up to target temperature than with my old tank, in the form of an 8 second or so ramp-up time while the heat exchanger warms up (this is timing it right out of the heater). On the plus side, the draft inducer in the unit is no louder than a modern fridge, and only runs for about 5 seconds after flow ceases. The digital thermostat is neat. Temperature of the water is stable even with variable flow. I can run a hot shower and faucet no problem with our rather frigid water here in Edmonton. I did not however try pushing the system yet to its limit.
Since I previously had an old, electric tank, I'm also looking forward to the energy cost savings. I would certainly do it again, but would consider a holding tank and recirculation system next time to speed delivery of the hot water.