Often, simple boot fitting is included when you purchase the new boots. If you purchase all the items together (skis, boots, poles), many shops will do the initial binding adjustments/safety check for free.
Standalone boot fitting can be expensive, but can also be well worth it, depending on how well the "stock" boot fits. A good bootfitter has an arsenal of tools available at his/her disposal to ensure a "perfect" fit. I've worked with Brian Deely before; he's one of the best (
http://www.pro-fitski.com/). (Disclaimer: Pro-Fit Ski & Boot Service advertises on DCSki.) If the boot already fits pretty well, you might be OK, but if it's not quite right, someone like Brian can make it perfect. The cost probably varies depending on how much work is needed, but a full bootfitting session (which takes 1-2 hours) might cost over $100.