Surely by this point you have made your long journey from China to Nepal, however for any future readers, I will do my best to answer this question.
With the new Lhasa Express railway from Beijing to Tibet, traveling in West China has never been easier. However, the railway will restrict the possibility of any true adventures and also prevent you from exploring the beautiful regions of northern and eastern Tibet.
Moreover, promotional tours sponsored by travel agencies often cost 5 or 10 times their true value, leaving yourself the tourist with empty pockets at the end of your holiday.
You can save thousands of dollars and also have a true backpacking adventure by doing it yourself. The first rule is to know that travel permits are NO LONGER REQUIRED to travel in Tibet. This is a myth that continues to be perpetuated by the local travel agenices to get as much of your dollar as possible. DO NOT purchase a permit because they are expensive and unnecessary.
To get to Nepal from China, you might want to follow a similar route that I traced, starting in Zhongdian, Yunnan province, through Kham, East Tibet, to Lhasa, then south down the Friendship Highway to Mount Everest and then into Nepal. You can do this either by bus, hitchhiking or an overland 4WD trip. The latter will be the most expensive option. The entire trip will take you a few weeks at the least, not including a Lhasa layover. You can find or post messages in local youth hostels and also the expat internet sites for other backpackers to join you on an overland expedition and share the cost.
Please read the following articles written by myself for more details about such a journey and the adventures to be had.
East Tibet
West Tibet
And as always, don't hesitate to contact me for any other questions about backpacking in China.