I agree with Martin, but only slightly. If you have a $15,000 car and spend $10,000 on parts alone then you have a $20,000 car. Labor (typically half of the bill if you're not doing it yourself) is never included. So if you paid someone $10k to install parts on your $15k car then it's only a $17,500 car.
If your parts were direct stock replacement parts like calipers, fuel pump, injectors, A/C or anything else that came stock on the car already I believe you've only maintained the $15k value of the car, not increased it. You just make it a more likely candidate to sell.