I bought an Atwood 6 gallon unit off ebay, far cheaper than any of the local RV shops were asking for them.
Mine is a gas pilot only model, I think I paid $240
Looks like the direct spark ignition models can be found on there for around $300.
I've actually been pretty impressed by my Atwood unit, when I get parked, I lite the pilot and I have hot water in about 30 minutes. Last trip, I lit it Friday and it ran till mid day Sunday and I didn't have to touch it the whole time. Insulation on the tank is pretty good, once the water is up to temp, the unit rarely cycles on other than when hot water is used.