This is what the Engineer described and quoted me, seems high to me and sense they may have smelled some misguided desperation.
In the garage, we will install (1) 100 amp 240 volt circuit to feed a new 100 amp sub panel in the garage near the tankless water heater (Main Breaker panel lives on the outside wall of the garage about 25 feet away).
We will come thru the back of the panel and install a surface mounted junction box on the wall. From the junction box we will pipe down the wall, over the door and into the new 100 amp sub panel. From the 100 amp sub panel we will install (2) 50 amp 240 volt dedicated circuits to feed the tankless water heater. The breakers will be 50 amp but the wire will be upsized one size to a #6 thhn per manufacturers requirements.
Total cost $1323.00