1999 Toyota Corolla Timing belt

Unless there's something unusual that happened, the valves get bent on an interference engine. You replace the valves for that, not the entire cylinder head or engine. That work costs the same regardless of the age of the car, but a used engine will cost less when it's from an older car that there's many of in the salvage yards. Your mechanic might be looking at that thinking it will save you money, but the labor cost will still be high too. Also, with a used engine, you don't know the history of it. You could get another one that the timing belt was neglected on and it's about to break. To be safe, you'll want to replace the belt before that engine is installed. Better to do that work on what you already have.