There really is that much difference. Raritan water
heaters are glass lined with replaceable anodes. When the anode wears out--the rotten egg smell in your hot water will tell you when it has--you just replace the anode, about a $50 part and a simple job...just pull out and replace the discharge fitting. And unlike older models that had steel
shells, today's Raritan water heaters have a polymer shell that doesn't rust. And they're still made in the US. A Raritan water heater
will typically last 15-20 years, even longer. Mine were 11 years old and still in good shape on one boat
when I sold it...21 years old and still in good shape on the next boat
. How old is yours?
and other inexpensive water heaters, otoh, have metal shells and anodized metal water tanks
...when the anodizing wears off the inside of the tank--which it will in a few years--the only solution is a new water heater.
So you do get what you pay for. Btw...I did a quick google
search and found Raritan 6 and 12 gal water heaters for under $700. Don't know if those included heat exchanger