Yes, just like any other service, you have to charge according to how much work is involved in actually cleaning a dryer vent.
If it ports out say, 8 inches from the dryer to the outside, we usually will discount it quite a bit. not much involved in that.
Bob, all the lint looks that color no matter how many of these we clean. you are looking at years of build-up in a dryer vent, versus maybe a lint-trap with colorful lint from a recent load of laundry.