If you want to do a complete strip and rebuild, then I suppose parts could cost anywhere between £500-£1000, depending on source and quilty (ie patten part, orig GM item or performance parts). This prices is just for parts, not inc tools, labour etc.
General parts you need are
1) new upper head gasket kit (head gasket, inlet gasket, camshaft oil seals etc) £50
2) head bolts (£10)
3) new inlet and exhaust bolts
4) may need new studs, dpending on how many came out and quilty of old ones
5) new valve stem oil seals if to want to take out the valves and re-lap them in (ie clean the diposts on the valve seats).
6) new bottom gasket set (sump gasket, oil pick up ring, water pump O ring etc) £50
7) new water pump £20
8) new rollers and tensioner (cost = depending on c20xe year)
9) inpect or recon or new oil pump depending on age and quiltyof unit (I had to get a new one in the end)
10) new crank bearing shells
11) new bottom end bearing shells
12) new piston rings
13) misc bolts for crank, end caps, engine mounts, H fram bolts etc
Got most of parts from www.autovaux.co.uk or I have access to trade club at any VX dealer via club calibra.