what is the cost of the ticket at Great Wall of China

Badaling, Mutianyu and Jinshanling Great Wall are three main great walls in Beijing. The Badaling Great Wall's and Mutianyu's admission fee are 45 RMB. Jinshanling Great Wall's admisstion fee is 50 RMB.
There are still other sections of the Great Wall in Beijing, such as Juyongguan Pass and Simatai. If you choose want to go any of them and I will tell you the more exact price of the section.

almost everywhere is price for ticket 45RMB (even outside of Beijing in Mutianyu county), although cable car ticket is usually more expensive than ticket itself