$100 is indeed a bit steep for a job of this nature, unless of course, as others have mentioned the following apply:
1. Late night/early morning.
2. Holiday.
3. High crime area.
4. Long distance. (in which case they should have told you that there would be travel charge)
Before you call anyone out to service anything you should always ask for a verbal estimate over the phone. If you are satisfied with the rate, you should also ask for a quote on-site to make sure it doesn't change BEFORE work is done.
Unlike many 'smiths my own rates do not change unless #3 or #4 (or both) apply. I find this satisfies customers a lot more to know that I have set rates that don't change on my mood. That said, I've recently adjusted my rates for certain things (such as lockouts) so that I am no longer the cheapest, but I'm also not the most expensive either.