Overstay, need an extension

hi, I will be unexpectedly be overstaying in PRC for 3 days. I have a 6 month multi-entry one, but it only allows me to stay for 30 days at a time. I will be spending some time in Urumqi, Kashgar, Lanzhou, Chengdu, and Kunming. Could I apply for a short extension in one of these cities (and which is the most lenient) and does the 500RMB fee still apply if I apply in advance? If it does, then I may as well fly out of the country for a visa run (which I want to avoid)

Thanks