Question for Dentists/Tooth experts...

Last month, a filling came out (the first molar right after the bicuspids on the bottom left-side of my mouth). So I just got back from my dentist appointment and he tells me I'll need a....root canal. He prescribed some meds for any pain, but the tooth isn't bothering me at all. Other than the noticable gap where the filling used to be, it feels just like any other tooth. My question is: what happens if a person elects to not go through with it and just leave the tooth as is? I have a feeling the answer isn't what I'm hoping for, but needless to say, I'm not excited about forking over $500 for a root canal. I will if I have to, but if I don't have to, I won't. I have a friend of mine that went through the same situation a while back and he still hasn't gotten a root canal and his tooth feels fine. Let me hear your opinions/experiences.