It is perhaps not a brilliant idea to ask about the costs of head gasket replacement, without first checking if the head gasket actually needs replacing!
I seem to remember the OP felt his head gasket had gone, because his coolant looked dodgy. The best thing to do would be to firstly reverse flush the cooling system, refill with the proper amount of anti freeze and water, and then see if there was any subsequent problems.
The K series engines can and do suffer with head gasket failure, but this can be linked to several other far more serious problems which effectively mean another engine in many cases.
It is possible to test an engine for head gasket failure with a kit which uses fluid, that changes colour in the prescence of combustion gases in the coolant system (takes about 5 minutes to do).
Chris