Foreign bikes are more expensive in China than outside. Dahons are cheaper there than outside, but may have inferior parts. My suspicion is that you may find deals around electric folding bikes as there is a lot of developmental activity there. As to regular folding bikes, the market may be challenging as the majority of population treats regular bikes as commodity and better bike market is for nerds that have excess money. I would start with share bikes - for this you need a local phone number (you may buy a SIM on Amazon ahead of travel) and a local bank account. Then I would move to a second-hand bike - talk to bike parking attendants around train stations. In the meantime explore the market. Shops with better bike offering tend to cluster around each other and may be adjacent to a cluster of motorcycle shops.