I was speccing to the 10-15 kwh range. Maybe you can get it for cheaper, but with the use case he described you're talking about a substantial continuous rated power and surge power. How many amps can your $10,000 50 kwh battery push? Also I'm including installation, warranty, etc. It would honestly probably be more than that because he'd likely need a more expensive inverter or 2 inverters to deal with that kind of load (I doubt his AC unit and dryer are DC).