What's the cheapest average to replace a 3.0 liter engine for a 2000 lexus gs30

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My Lexus GS300 overheated while my friend who I let borrow my car for 3 months was driving it & needless to say my friend doesn't know much about vehicles. So it has an automatic shut off when it overheats so he let it cool off then added 1 gallon of water let it cool off completely the rest of the way. He didn't know that once you start the car that the valve opens and you have to add more water so without doing that he started driving down the road again made it about 2 miles and it overheated again and now wont crank. A friend of mine's uncle is a mechanic and checked it out for me free of charge before I had it towed into a mechanic shop and let know that yes it is a blown head gasket. I have also been told I'm probably better off just replacing the engine.