I don't see why that idea would be expensive though. It seems like something that could be done for $20, and perform as well or better than a large plate exchanger. The biggest downside is the floor space and low SAF.
I doubt you are going to find everything for $20. Copper loops large enough to have any significant heat transfer aren't going to be cheap. Still even if it was free this is the water my family drinks. Not sure I want to be hooking up some salvage hot water heater of unknown origin or quality.
I fully concede that a flat plate exchange on the cold water inlet doesn't recovery 100% of theoretical energy but is does recover a good portion, is cheap and simple and doesn't require potentially contaminating the water supply.
If I went the storage tank w/ heat exchanger route I would look for something like this:
https://www.signalmarine.com/p-51907-atwood-eh-20-electric-water-heater-wheat-exchanger-20gal-110v.aspxIf someone made something like this without the backup electrical heat it likely would be cheaper.