I think you got your answers in the other posts, but just in case, check out
www.kbb.com to get a general idea. Remember a car is worth what a person will spend on it, so this isn't an exact rule, it just helps in determining an average asking price. Also remember mods don't add 'value' to a car, it is still worth the same amount even if the previous owner invested 30k into the engine. You may see that as a benefit and be willing to spend more money on that car, but overall it doesn't add value, it is usually just hte previous owner attempting to get back some of the money he dumped into his car.