Travelling to Shanghai from bkk what Airline to use and what is the cost.

Go up there reasonably often (work). The only direct flight I would even consider is Thai, who usually fly Airbus 330's on that route.

If I'm going via Hong Kong I take Cathay and then Dragonair.

Not for one minute would I recommend you consider one of the Chinese airlines. It's probably China Eastern flying into Shanghai.

No idea about cost, sorry, company pays my fares.