Depends on the rifle. If it can be re-chambered without removing the barrel, like most bolt guns, then it's only about a 10-minute job, and $100 would be more than adequate.
If it's something like a Ruger #1 though, the barrel has to be pulled off, reamed, and re-installed, so that's a lot more work.
What is it?
Hows this a 10-minute job?
