Check out World By Bike
They did a big stretch of China.
I am planning to travel from Pakistan along the KKH into China via Kashi across to Beijing or Shanghai in June/July 08.
I have been quoted the following:
All entry/exit paperwork 4500 euros
Guide costs (for approx 2 weeks) 1500 euros
Car hire costs for the guide 3600 Euros
Kind of put a halt to my plans, unless there are others wanting to share the costs? Almost makes you want to drive up to the border and try your luck!!
Hope this helps...