You can find local guides and drivers in China for local areas, on short notice and at reasonable cost. You cannot find local guides in the rest of China for Tibet--they wouldn't be authorized to take you there. It is possible to get to China then use an agency to set up your tour, but due to the timing of the processing, you need to allow 2-3 weeks in China prior to Tibet, to allow it to work out. And if you go at a time when there are no other tourists to group you with and share costs, you'd have to pay more. This is why most foreigners visiting Tibet plan and make arrangements ahead of time, sometimes crafting the actual dates of the Tibet tour around something already set up or in process, if they need to share to make the economics viable.
The Chinese have figured out that Tibet is a cash cow for tourism and they are going to milk it for all it's worth. While prices are normally far higher than in eastern China since everything has to be flown, trained, or trucked long distances (food, fuel, supplies, etc.), the premium over and above that is breathtaking and makes Tibet one of Asia's most expensive destinations on a per-day basis. The restrictions on foreigner access and requirement to use authorized providers and non-public transport, makes it all the more so. (Though note that even for Chinese citizens who do not require a permit and who may take public transport, it is considered an expensive destination).
Edited: 7 years ago