We charge the same for both shop and road work. I know some charge more for road work.
The big issue we always face is travel time. It really comes into play for customers that are over an hours drive one way. They usually have a big issue paying a lot of money for someone to "walk through the door". Some people call it a zone charge. Some people charge travel time from the previous job. We charge all customers as if we were driving to and from our shop.