Repair the old brick wall or build new?
If your old wall stands straight and the bricks are sound but the mortar is crumbling out of the joints, brick pointing is usually all it needs, and I'll tell you if even that can wait. When a wall leans, has cracked through at the footing or the bricks themselves are breaking up, patching only buys time, so we take it down and build a new one on a proper footing.
What sets the cost of a brick wall
Length and height decide how much brick and mortar go in, and a taller wall needs a bigger footing that may need reinforcing. Sloped ground means more digging and stepping the footing down the hill. Salvaged brick carries its own cost in sorting and checking. The cap style, how far materials have to be carried, taking out an old wall and any permit work all add to the total too.