Air China, PVG-PEK
 

Am debating whether to pay $138 for Y or $207 for F. Will there be any service on this 90 minute flight?

Can I get a cheaper ticket by purchasing it once I have arrived in China? The coach fare seems sort of steep.

Can't help you much with fares, unfortunately, but my experience with CA on the PEK-SHA route in coach was quite good. (You may wish to refer to my trip report.) The aircraft was an international configured 744 with plenty of legroom, and a rather generous meal (it was a morning/breakfast flight -- Japanese noodles, turkey sandwich, fruit, cake). Service was pleasant. It seems to me that paying more for F (even just $60) may not be worth it for such a short flight, but not having flown F, I don't know what the comparison is on this route.

The difference is not really worth it for such a short flight on CA. However I regularly pay the difference between business and first when flying between Shanghai and Hong Kong with Dragonair....that's a different class altogether.