$5k is way higher than $1k, that’s like $4k profit. And the motor cost $4k which is less than what you can sell the car for afterwards so that means it is worth doing. And the wrecker is just a scammer, they are trying to give her $1k for a car she’s going to be able to sell for $5k. Obvious rip off, they just want to put a motor in and get all that profit for themselves.

Your MIL knows what’s what. She’s not going to be fooled by your fancy-pants ‘profit=5000-4000-150’. That’s just ivory tower university maths, it’s not how the real world works.

Sorry, apparently I can explain the thought process, but nothing can make it actually make sense…