Really depends on the vehicle. I was able to pay for college by doing exactly this with DSMs and Kawasaki Jetskis.
DSM stock parts are free or damn near. Aftermarket stuff still brings 50% of new or better.
Example, buy a modded DSM with the following:
20G turbo, 650cc injectors, FMIC, DSMLink, 3" turbo-back, gauges, aftermarket wheels, AGXes and Pro-Kits. (99% of modified DSMs are similar to this car)
Sell the following parts at the following prices:
20G Turbo $500
650cc injectors $200
FMIC with aftermarket BOV and piping $300
DSMLink with EPROM ECU $300
3" Turbo-back, $200
Gauges $150 (say they are nice, GReddy or Defi or similar)
Aftermarket wheels $400
AGXes/Pro Kits $400
Install the following parts and pay the following prices.
14B Turbo $75
Stock injectors $40
Stock FMIC and piping $100
Stock non-EPROM ECU $75
Stock exhaust $free
Stock gauges $free
Stock wheels $200
Stock suspension $free
Stock exhaust and suspension are listed as free because its common to see them given away and they carry no value other than their weight in scrap.
Things like a built trans, clutch, fuel pump stay in the car, these are selling points anyway because they can be a pain in the butt to do.
So you clear $1,960 selling the parts off. You can buy a clean modded DSM for $4k and sell a clean stock one for $3500. That nets you $1460 cash money.
Key to doing this is having a few parts cars and keeping in the brand, say 90-94 AWD DSMs only. The wrecks can donate parts to fix up the nice cars. If you are stuck buying all you stock parts from forums and boneyards, its tough to make real money. If you have a few parts cars at your disposal, are good at cleaning and detailing, and don't mind Ebay, you can make some good coin.