Flight was under £100 when I did it in May. There is not much competition on this route due to Government restrictions, so the price is higher than might otherwise be the case but is still not unreasonable.

Do fly - if only because you get to experience The Worlds Fastest Train (the Maglev) from the airport in PVG (albeit that this is not cheap, but you've got to do it once).

24 hours on a train is too long, although if it was an overnight then I would recommend it as hotels in HK and PVG are not bargains.

Note that some online booking systems like Expedia do not, for some reason, show all of the HKG - PVG flights. There is roughly one an hour on Dragonair or China Eastern.