Cost depends on the project, but for simple projects it adds about $75+ per color to the project. We don't like doing it because it can be very disruptive to our workflow and may also cause delays in your order, but we will if it's a non-negotiable.
Why: it's the only objective way for us to target. Pantone books are good because they try hard to keep their swatch books consistent. They're not perfect though. The same color between pantone books are slightly different, and most people don't spend $180 a year to keep their book color books accurate (including us). Color fades over time.