What is the cheapst way to Beijing airport from Beijing Regent Hotel?

From the airport, you could take shuttle bus Line 3 and get off at Dongzhimen. From Dongzhimen, you should take subway Line 2 to Yonghegong station and transfer to Line 5. Finally get off at Dengshikou station, you will walk to the hotel from Exit C. The shuttle bus fare is CNY16 and the subway fare is CNY2.
Ps: after 24:00, you should take shuttle bus Line 2 to Dongsitiaoqiao and then hire a taxi driving to the hotel.