replacing head gasket

Can someone please tell me if a price in the region of 600 to 700 is a reasonable amount to expect to pay a garage to replace a head gasket to repair our car.
We have a P reg Astra 1.7 TDI with only about 65,000 miles on the clock and last night it died with a spectacular bang and a smoking exhaust. The garage towed it away this am to have a look at it and apparently it is the 'head gasket'. Once this is replaced the car will then have to be pressure tested? (sorry not at all mechanically minded). It seems a lot of money to me but possibly not a straightforward job.