I want to visit to JiuZhaiGou from Chengdu by bus but want to visit Huanglong and etc

To JZG, you can take long-distance bus from Chengdu Chadianzi Bus Station directly. The bus schedules are from 08:00 to 19:10 with the ticket fare of CNY 157 per person. Then walk to the scenic area.

After touring around, you are suggested to hire a private car to get to Huanglong Scenic Area. Generally speaking, the fare is CNY 500 or around for the single trip.

500 CNY per person?

No, CNY 500 is a single trip fare for one private car.

Perfect Edward, do you know how to book this trip? We are 4 adults and 3 children.

As I know, you can easily find the private car when you arriving at there. And remember to negotiate the price with the driver before getting on the car.
Or you can also join in a tour to visit these scenic areas. TravelChinaGuide is a recommended one. They have responsible tour guides and customized routes. If interested, their contact information is traveltravelchinaguide.com.