Such a positive outlook you have! Prostitutes and drug dealers have to be of very high value based on your concept, since they offer a lot of joy for a relatively affordable price. What about those who alleviate suffering? Doctors are valuable, sure but sometimes the medicines and procedures they do can cause just as much harm as good.
Also, it wouldn't be hard to imagine a serial killer who targets his victims based on this premise - they're causing suffering and he's putting an end to it.

Its my opinion that we can't estimate the value of human life because we would be reducing the person to an object and worse - we would judge them based on our prejudice, our mental concept of them and not their real self. We don't know and can't know a person's heart - that's why we can't judge and value other people. Be honest, try and put a price tag on your heart, what is the price of your soul?