Sure you can. Maybe not always gem but even AAA isn't hard to find. Price would depend on when and where you go. For timing I'd say after Chinese New Year if you're buying from retail outlets, assuming you even have a choice as to when to go. That's when business is least brisk and they'll cut prices just to make a sale even if they don't make much. I wouldn't pick the Golden weeks (even if they are trying to abolish it). For metro Shanghai's Hongqiao pearl city is pretty good one, Weitang in Suzhou and finally the biggest would be in Zhuji (Shanxiahu). I don't think I want to go all the way to Zhuji until the jewellery city is complete. Suzhou is as far as I'd wander. It gets cheaper the farther away from the city you get. If you speak the language it's ideal, as you would not require and interpreter and can ask for exactly what you want. I agree the terms to use would be luster, roundness, and lack of blemishing. Have a reference strand with you to compare with until you're confident of ascertaining quality (well, just in general for the non-GIA certified members here anyhow). Even if you don't speak the language, it just takes a little longer to get the point across (there are usually other helpful tourists around - if I happen to see someone asking for something at a pearl store I'm happy to help at least) and some of the younger store owners actually speak English. Calculator talk definitely gets the point across. Beats getting an interpreter because sometimes they take a cut as well.
That doesn't mean most of what's on the floor/display isn't junk. You just have to ask for what you want, that's all. And be careful about pricing - don't look too keen to buy. They usually slash prices themselves if you're hesitating and trying to get away.
Now you've got me itching to go again ...