Gifts for Chinese hosts?

The holy trinity of Chinese gifting is liquor, cigarettes and local delicacies.

In the $15-20 bracket you're presumably not looking to bribe anybody, so a nice bottle of California wine might fit the bill, although they're fragile and a pain to transport due to liquid restrictions. For local delicacies, things like chocolate or candy are pretty safe, and much easier to carry. Extra points if it's got something 'healthy' (according to Chinese medicine) like ginseng in it. This handy infographic lists most of the do's and don'ts, although it's a bit geared towards people already in China (unless you were thinking of bringing pu-erh tea or longans...).

In all cases, packaging matters, and it's actually a bonus if it's as tacky/obviously-foreign as possible, meaning draped with eagles and flags or packed into a Statue of Liberty tin or whatever. The reason is that your gift is likely to parked somewhere visible, so your host's visitors can see it and be impressed by tribute brought to him from far-off foreign lands. (Probably not that big a deal in your case, where your hosts are academics who presume do this sort of thing all the time, but worth keeping in mind if visiting people who don't generally have laowai guests.)

I haven't traveled to China, but I have traveled extensively through Thailand, India, Indonesia, Pakistan and Nepal. In my experience, the locals really appreciate small American-made things like chocolates and candy. A bag of Hershey's kisses made me the favorite person among all the kids when I was visiting a friend in India.

Key here is to buy something obviously American or American made. For my Indian friends, I bought some quality American-made pens which they appreciated a lot. On a lark, I bought my Indian friend's older brother an American-flag wallet, which, to my surprise, he still carries around to this day.

If buying candies and similar items, I recommend buying something that's not easily available over there. This means no Mars bars or Snickers. Instead, get handmade chocolates (more expensive) or cookies. In my case, Pepperidge Farms' Milano cookies and Pirouline Wafers were especially well loved.

My Indian friend's dad also requested me to bring some American BBQ sauces. Apparently, he's big on the grill and these sauces aren't readily available there.

A lot would depend on your familiarity with your hosts as well, but food items are a safe choice. Other items like wallets, souvenirs or other nick-knacks might be construed as 'tacky' by some people.