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