Salt-belt Corollas (Midwest, Northeast, Canada) are prone to rust on the lower rocker panels, inside the rear wheel arches, and along the bottom edge of the doors. Surface rust is cosmetic; bubbling or perforated metal is structural.
Crouch at each corner and inspect the rocker panels end-to-end. Press on the metal — solid is good, soft or crinkly = rot underneath. Check inside the rear wheel arches above the bumper seam.
Est. cost: $300–$1,500 cosmetic / $1,500–$4,000 if structural rockers are affected
Source: Salt-belt Corolla owner documentation