$400 complete is possible, if you "build it"... and that is just really 'assembling' one. Might have to 'assemble' a lower, but sometimes complete lowers are really cheap. Then buy an upper, attached with 2 pins... "BUILT!" 🤩
Prolly be more in your state, depending on certain restrictions... Cali is really bad, they have the 10rd, non-detachable magazine rule, no pistol grips, among other things. Funky looking lowers on those.