1

If you depart from the same airport you will never pass customs/immigration and do not need a visa.

2

If you do you will get 24 hours or is it 48.

3

Foreign crewmembers or passengers, excluding citizens of the USA (except for transit in Shanghai) or UK, do not need a Chinese visa if they take a direct transit in China by plane for less than 24 hours without exiting from the airport.

Citizens from the following countries do not need a Chinese visa to transit in Shanghai if they enter and exit from Pudong or Hongqiao airport, stay for less than 48 hours, and hold their valid passport, visa of the destination country, and on-going air ticket with departure date and seat confirmed:
Australia, Austria, Belgium, Canada, France, Germany, Greece, Italy, Japan, Korea, Luxemburg, the Netherlands, New Zealand, Portugal, Singapore, Spain, and USA.

There's more about transiting if you'd like to do a search here.

4

In July 2010 I was heading to China from Canada (am a Canadian citizen) to start a 2 year contract with an international school. I wanted to take a one way flight to China (that I would be reimbursed for later), and then had a separate flight to Tokyo five hours later. My work visa wasn't ready on time, and I had read that you don't need a visa for 48 hours transit, so I just went as is and planned to get a tourist visa in Tokyo. When I went to get on the plane in Toronto, I wasn't allowed to board - even though I had proof of my onward ticket and had read on an official website 48 hours for transit was allowed. I had to change all my tickets. So just be careful!

5

I've taken the flight from London to Melbourne at the start of 2011 via Shanghai Pudong before. The planes don't have TV screens but otherwise is OK. I didn't know that I had to pass customs and immigration (which was a pain) and then go back through immigration again immediately afterwards, but that's what happened and I received an on the spot transit visa.

6

Thanks for all the posts
does transit visa cost something?

7

No it was free.