A second hand bike of $80 - $100 is probably obtainable if it is a low end bike.  We have seen new bikes around $300 in China, so the second hand low end bikes in China probably cost next to nothing.  On the other hand the high end bikes market is China has been expanding rapidly recently, so these low end bikes may disappear within a few years from the major cities.

However I would be surprised if any of our members posses such low end bikes when a single bolt for us enthusiasts could cost upto several hundred dollars.

Wish you good luck.