Tibet costs can vary widely. While those agents who cater mostly to western tourists can charge you a lot and force you into packages with 4* hotels, you may be able to find agents that mostly cater to those from mostly Taiwan. Last year, I used one of these agents, and the cost was very reasonable:

22,000RMB for a Landcruiser, driver and Tibetan guide (non-English speaking) for 15 days outside of Lhasa (including gasoline, but nothing else); and the TTB permit for one (myself). And they let us book our own hotels in Lhasa and just leave us alone. But whether they can communicate well enough in English to arrange with you is another matter.

Lodging in Lhasa is fairly cheap - our clean Tibetan-style guesthouse in the old city (run by Han Chinese, not Tibetans) charge us only 100RMB a night per double room last June. With private baths, breakfast and free wi-fi that's actually fairly fast. But outside Lhasa, hotels are pricey. "Real hotels" in cities like Shigatse or Nyingtri are 300+RMB a night (about double of what similar places in Chengdu or Xining charge); or you are left with dorm-style guesthouses, often with only shared bathes.

Train ticket cost is another huge variable. We rode the train out of Lhasa to Xining at face ticket cost, as our nice Tibetan guide sent his mom to the Lhasa station to get them for us at 10 days out. But other agents were tagging on surcharges between 100RMB and 1,000RMB a ticket; depending on date, direction of travel, and origin/destination. 1,000RMB/ticket surcharge was for Xining->Lhasa soft-sleeper during July and August, 2010.