The real prize a professor can receive is the student's gratefulness.
"Thank you. But not for your lectures, and not for the time you invested to help me with the subject. Thank you because I thought that I had mistaken in choosing my career. I thought that I was loosing my time; but you have lighted my love for my profession and I have found my vocation. Now, I know that I am in the current path. I just want you to know that I will never forget your lectures."
This is an anonymous email I received after one final exam. These words are my best prize.