LA is a very big place, so there is a big variance between the coast (e.g. Santa Monica, Venice, Marina del Rey, South Bay) and the inland or valley areas. Usually the weather reports are almost like clockwork in an X/X+10/X+20 pattern: 70's at the beaches, 80's inland, 90's in the valleys.

If you want to stay cool and not too dry, it's not too expensive to live in the southern areas or even Long Beach.