What is a fair fee for a core build-up?

I just had to have a root canal on my #3 tooth. There was a crown in place. The root canal specialist was able to drill through the crown, do his work, then sent me back to my dentist to repair or replace the crown. My dentist said the crown was in good shape, that he would do some core buildup, then fill in the hole the crown. My question is this: What is a fair fee for a core build-up?