Repair or rebuild a retaining wall?
If an older wall has loose stones or tired mortar but still stands straight, repair usually makes sense, and masonry restoration can bring the face back. Once a wall leans, bulges or pushes water through its joints, the trouble is usually behind it, in the drainage or the base, and patching the face won't stop that push. That's when I'd take it down and rebuild it with the drainage done right.
What drives the cost of a retaining wall
Material makes the biggest difference, because prices around Santa Barbara vary a lot between boulders, cut stone, poured concrete and interlocking block, and stone laid by hand takes the most time. After that comes how much soil we dig out and haul away, how easy it is to get equipment into your yard and how much drain rock and pipe the wall needs. The finish you want on the face plays a part too.