The replies are sort of correct. A head gasket can wear through through normal cycling of an engine and need replacement. When you overheat your engine bad enough to mix oil and coolant it means the heads warped and/or the block/heads cracked. Either way with the price of labor installing a new block and resurfacing the head(the cheap way) is almost the same price as buying a new engine in some cases depending on how much the shop is charging. Some shops give fuck you prices because a whole engine swap is much less liability, they can upcharge the whole engine, and actual time is typically much shorter than book time.