Is there a major difference on flight airfare of Lax to Beijing in May and Sep?

I plan on having 2 weeks in China (Beijing, Xi'an, Shanghai) however I am looking at two different dates. First 2 weeks of May or last 2 weeks of September.

Right now plane tickets fomr LAX to BEI is around $850 for May and $900 for September. Is there a major difference between these two dates? Also, I have seen plane tickets for as low as $750 between LAX and BEI. Is $900 a decent price or should I wait for it to come down a bit if I chose to go in September?

Thanks