what is the cheapest way to go to Haidian district from the Beijing airport?

Haidian district covers a large area, which specific area do you refer to?
As far as I know, the airport shuttle bus line 4 take passengers to Gongzhufen, which is located in Haidian District.
If you can tell me more detail information, I may tell you how to transfer to your exact position.

Near the Chinese Agriculture Univ. Thanks bunch!