We’d like to give a precise answer, but fixing rust can cost anywhere from $15 to thousands of dollars, depending on how bad it’s gotten and where it is.
On the low end, light surface rust can often be removed using WD-40 and painted over using a paint pen matching your vehicle’s exterior color code. On the high end, perforation of a structural component can require the entire thing to be replaced, costing well over $2,500.