Well, I did the research into the types of tankless. The Takagi makes bosch mid to high end. Paloma makes the rest. Bosch has some board problem that took out a couple people I read about. So they were out of hot water for 4 days. So I nixed those.
Rinnia(sp) and some n one were the others that looked good. So I found an installer for the rinnia one.
Basically I would need to run new gas line from where it tapped into the house. But that depended on the line I had to the hot water heater. Also the vent had to be some special vent piping that ran about 100 bucks a foot. Regardless, his ball part was 2500 - 4500 without looking. He said that was the average for the few he retroed. His company mainly did new home installs for these, so they didn't have that issue. Could be better, could be worse. I didn't want to pay 40 bucks to find out since the 2500 is out of my ball park anyway. Figure even with gas going up, worst we do for the hot water heater is like 50 or 60 a month. Best case we save half that. Makes it about 360 a year, so it takes 10 years best case to save our money. Worst case is like 25 percent. So it makes it 20 years. They aren't even rated that long. So say it falls in the middle to 15 years to save it back. Then it is time to get another. Not sure I see the benifits for a retro fit. New install, I could see it all the way.
Thanks for the other replys. I will do some research on those.