Depends on which engine, which valve, how much (if any) secondary damage has been done and where you get it fitted!!
An intake/exhaust valve done at a Vauxhall main dealer garage will cost many hundreds of pounds, an independent garage will usually be cheaper but still a few hundred pounds . If a valve has dropped in to the cylinder there will be so much damage you'd be better to fit a replacement engine. Depending on the value of the car (and it is just a ten year old Astra) you may be as well scrapping it.
However if it's just something like an EGR valve for a 1.6 the part will cost about £50 and anyone who can work a spanner can fit it in ten minutes.
Without FULL information about the vehicle and the problem it's impossible to answer.
fred, March 2012
