From the rules:
ATP World Tour Finals 1,500 for undefeated Champion (200 for each round robin match win, plus 400 for a semi-final win, plus 500 for the final win)
Here is my understanding some of the ways it could play out.
IIRC Murray leads by 405 points as of now. If he loses on Friday and Djokovic wins tomorrow, and Cilic wins tonight, he will still lead by 205 points.
If they both then lose in the semi finals, Murray would still be no. 1. If Djokovic loses tomorrow and Murray wins, then Djokovic beats him in the final, Murray is no. 1 as Murray would have an insurmountable 605 point lead going into the final.
http://live-tennis.eu/ has the up to date points.