You will get many opinions, but here is mine:
1. Find out what the yards typically pay in your area- get a REALLY GOOD IDEA over several months, several yards, and seasons if possible
2. I will not pay more than I can get off JUST THE BODY- so if the curb weight is 2300 lbs and it is paying $200/ton. I'd make $230 on JUST THE BODY.
3. I will take alum rims, cat, battery, alternator, and wiring harness out, I will also pop the tires off those alum rims and sell them if they are good.
4. Obviously those above mentioned items take weight off the vehicle.. so I always have a bunch of scrap steel accumulated to pack the vehicle with and more than make up the weight in parts taken.
This simple rule of thumb pays me well. I do not have the room, tools or ability, to gut a car to its frame as some folks do. With those means- you could buy cars at $4-$600 and be ok. What makes this difficult (scrapping cars) is the amount of variance in them. You find a Mercedes for $400- Snap on it- the cat alone will usually pay you over $200, set of Alum AMG rims on it- CL them instead of scrapping them. I just saw an 87 corvette on my local CL this weekend for $500- if not for my bum thumb- I would have snapped that car up in a second- Corvette folks are fanatical to have CORVETTE parts to keep them OEM. I could have cut the cat, batt, alt, and sold the car even with those parts gone for probably 4X my money to a corvette owner for a parts car. It is all relative.