Yes, that is decent. Occasionally SWA will run a Ding to Ontario that is lower (usually around $178 ow when it turns up), but in general, STL to Southern California is not a cheap prospect, and LAX is normally the most expensive of the 4 airports to fly into. (That is, choosing from LAX, ONT, SNA and BUR; Burbank is probably the second-cheapest route; it also gets last-minute sales.)
FWIW, I just checked the SWA Low Fare Calendar for May/June, and there are some flights available to ONT at $144 on Tuesdays and Wednesdays. Midweek flights between STL and ONT tend not to fill up, so they often go onsale last-minute. Fares for later in the summer are all fixed at $247 at the moment.
Unfortunately, though the prices are lower, the flight times to the Southern California suburban airports tend to stink. When I have to go to Southern California for business I almost always find that the best way to handle the timing is to overnight en-route outbound. You can get some very affordable rates on airport hotels in Las Vegas and Phoenix, and there are flights out of those airports to Southern California that will get you there before 7 am. (Also, both of those airports often have sale flights out of STL.)