Hello China travellers.
I would like to give my opinion and comments as I've just performed and enjoyed the 3km trek from Jinshanling to Simatai Great Wall:
-It is not dangerous at all.
-Grab some fruits and water in Beijing if you don't wanna pay extra costs for it at the great wall. (10RMB for water, coke or beer cans)
-Take the 980 bus to Muyin and then bargain a taxi (after dealing a couple of minutes the best you can get is 70RMB to Jinshanling - shared ride)
-Pay 50RMB for entrance fee at Jinshangling and then 30RMB for a cablecar ride if you don't wanna make the way up to the GW. (no foreinger students cards accepted here)
-After the first 1.5km you will be asked to pay the Simatai GW entrance fee: 40RMB (foreinger students cards are accepted here and you get half price dscnt).
-Almost finishing the way you'll cross a bridge and after this asked to pay 5RMB.
-Way back to beijing: you can bargain a minibus up to 70RMB at Simatai campsite. This makes me think that you can get the same from Beijing to Jingshang if you make a good job.
-Note: I've really tried to find the local 8RMB bus before and after the trekking, and was not possible to get it, seems not to exist.

Enjoy it, is a great experience if you made the way up to Beijing.
Good luck!
Gejo.