Exactly.Same as auto repair - sure you can find anything cheaper online. No surprise there.
Does that cover what the cost of having the tank on hand was by the supply house the plumber got it from? Does that price cover the plumbers overhead? Warranty work if something fails, doesn't work correctly, etc?
Finding things on cheap online never covers what the actual cost of getting work done by a professional is.
The professional has to buy from a supply house to be able to offer a warranty. Not Amazon. That’s not a meaningful comparison.
Call the supply house, see what they charge. Then you have a worthwhile data point.
If you don’t like the price - DIY.