Ours sell for $425. They should not be vented out the chimney but direct vent like a dryer by venting it out the roof you probably will shorten the life of the heater, because of condensation leaking back into the unit. I'm glad it worked for your floor plan. We also sell smaller models that you can hook up to a propane tank and take a warm shower while camping.
The installation manual for mine said I must have five feet (minimum) of vertical double wall pipe for proper draught. Do you not call the double wall pipe that goes up through the roof a chimney? Whatever you call it the one on the water heater that we had was not sufficient and it had to be replaced, including putting a bigger hole through the roof.
The tankless unit itself cost about $425. The rest of the cost was installation (and parts -- my LP company wasn't giving away the two stage regulators ;-), and all that double wall pipe adds up. ) It took a lot of time to install, and the installation had to be by someone with a mechanical license or it voids the warranty.