Short trip to mexico?

trying to plan a short 4 day get away with my girlfriend sometime in early august leaving from california. trying to be as cost effective as possible and i know it can be difficult during high season…any suggestions as to where to go? i think the quality of the beaches would be the most important factor in location. any ideas of how to keep cost around $1000 total, give or take a few hundred of course. thanks so much fluther nation!!!