Tibet uses the Chinese currency---RMB, or CNY. Detailed currency exchange guide please kindly check this link for reference: www.boc.cn/sourcedb/whpj/enindex.html
ATMs are available in Lhasa and Shigatse area. The Bank of China accepts Visa, MasterCard, Diners Club, American Express and Plus. The Agricultural Bank accepts Visa, Plus and Electron. Check before trying your card as many ATMs can only be used by domestic account holders. The maximum amount you can withdraw per transaction is RMB2000/per day.
Money Exchange & Credit Card
Bank of China is the only place to change foreign currency and travelers cheques. The currencies of Australia, Canada, the US, the UK, Hong Kong, Japan, the euro zone and most of the rest of Western Europe are acceptable at the Lhasa Bank of China. Some 4 to 5-star hotels in Lhasa also have exchange services for guests. The official rate please kindly check this link for reference: www.boc.cn/sourcedb/whpj/enindex.html
Outside of Lhasa, the only other locations to change money are in Shigatse, Zhangmu, Purang (cash only) and Ali, and at the airport on arrival. Other than that, credit card acceptance is rare. This will undoubtedly change in the future but, right now, it means that you need to use cash for most transactions.
Lhasa Central Branch of Bank of China is the only place to officially change RMB back into foreign currency. You will need your original exchange receipts. It is not possible change RMB into dollars at Lhasa Gongkar airport.
Moneychangers at Zhangmu (by the Nepal border) will change RMB into Nepali rupees and vice versa. RMB can also easily be reconverted in Hong Kong and many Southeast Asian countries.
Living cost of Tibet is much cheaper than western countries, might be slightly higher than some Asian countries such as India or Vietnam. So, the cost is relative depending on your standard. The common meal cost in Tibet is around USD7/per person per meal.