how can i travel from Leshan or Emishan to Jiuzhaigou with the shortest cheapest route?

No direct transportation is available.
You need to take a bullet train from Emeishan to Chengdu first, which takes about 100 minutes and CNY65 for a 2nd seat. Then take a taxi to Chadianzi Bus Station, which takes about 10 minutes and CNY20-25. After that, you can take a long distance bus to Jiuzhaigou, which is available from 7:00 to 12:00 with interval of 1h, and it takes CNY246.
By the way, Jiuzhaigou is closed temporarily because of the earthquake happend on Aug. 8th.