So, let’s say 2.5k on the PA, 1k for the controller, 1k for laptop, cables, light, stands. Total about 4.5k (conservatively). Rental fee 4% = 180. Hourly rate at a minimum 50 (only if you want to give a big discount because the family connection etx), for a total of 350. Not talking about time for setup/breakdown and transport, that means about 530 at the low end of the spectrum imho.
So, they should have hugged you and accepted the 400 bucks. Frankly, even for “friends” deal I don’t go below 600 anymore and that is for a 4 hour set. Usual prices range from 1000 up.