You can also check the fee of subway at station.

本线（1，2号线）票价3元
…The fee for this line(1st or 2nd line) is 3RMB per a ticket.
换乘八通线票价4元
…The fee for this line(1st or 2nd line) is 3RMB per a ticket.
换乘13通线票价5元
…The fee for this line(1st or 2nd line) is 3RMB per a ticket.
请您当面点清票款
…Please make sure that you have a right ticket and change here.

I recommend you check the line in the Beijing Subway System website.
http://www.bjsubway.com/chinese_ver/cczn/dtxlt.asp