Would depend on a number of factors - engine type, what sort of way in which the gasket has failed (ie, just an oil weep or more serious cooling system issue - whether the head needs skimming (normally not the case with most MB heads I understand) etc.
I guess somewhere between £600-1000 would a fair ball park figure for most average garages.
I am looking for another car, just not sure what yet. I loved my last c230k, but not many of these around without the chequered interior, much prefer leather or the younger cloth pattern. OR a C280 might fit the bill? Or something else entirely, I'm not really sure what I want, which is hard because I don't have a clue what to buy.