Lots of good points in here, especially from Animal IMHO.
Couple of things i reckon need to be stressed however:
There must have been a reason for the gasket to blow in the first place.
Use new head bolts! You dont want the new gasket to blow again straight away!
To answer you directly Intrepid, prices for head gasket replacement are something like $1500-2000 Australian...
And i would be pretty skeptical about buying a car, however cheap, with a blown head gasket, as there must have been a reason for it to blow in the first place
What sort of car/engine is it?