No trains are available from Chengdu to Jiuzhaihou. You can take long distance bus or flight. By bus, it takes around 8hrs and costs about CNY140 per person. Many flights are operating on the route and it takes around 40mins. For the specific airfare discount, you could search on the page with your exact departure date: https://www.travelchinaguide.com/china-flights/chengdu.htm