We don't have a fixed number to quote you here, and we're not going to invent one just to fill the page. What drives the cost on a Norwalk job is usually the roller type the door actually needs rather than a flat per-door rate - steel rollers with sealed bearings cost more per unit than basic nylon, and a door built around 1957 is more likely to need the steel kind if the track hasn't been upgraded since.
The other variable is how many rollers are actually bad. Some owners replace all ten on a double door in one visit because the rest are close behind the one that failed; others only want the two or three that are grinding or cracked swapped out. Given the age of a lot of Norwalk's housing stock, doing all of them at once often makes more sense than coming back in eighteen months for the rest - but that's a call we make at the door, not over the phone, and we won't push a full set if the others still have life in them.