how much is going to fix my corsa 1.6 sport 2004 model is mixing oil n wate?

With old card back in 50's 60s early 70's you could just change the gasket and as long as the cylinder head and block were clean up the main thing then was to tighten bolts in the cylinder head in correct order and correct torque setting or it would just leak again.
Now on most if not all the cylinder heads need to be skimmed therefore pushing up the cost to around £600+ but I may be wrong you car may not need the cylinder head skimmed. Shop around. Ask around locals and friends a garage they recommend.
Don't drive it you could bugger the engine up as oil floats on water and you could be getting a wrong level reading or the dash indicator might not show lack of oil. Also think about cam belt change while its stripped down. Its cheap same time repair and less costly if the belt breaks. Usually 60,000 miles or on average 3 to 5 yrs.

Bob
January 2015