The reason I saw this as a mistake, I have seen it before. It started with the guy digging the hole went a little deeper to get to good soil in that corner. The framer ( inexperienced) just continues with 4 ft high forms. What hard to beleive here is, it is check be the builder, the form rentle outfit, the engineer, the city inspector and the servayours. They put treated lumber against the framing on the inside and poored the floor to that.
Joe could be right but I think there would be more to see as far a damage.
To answer the first question. Hard to guess prices but, remove, much of the garage floor, remove some of the driveway, backfill the garage and driveway, form up driveway, poor new concrete?
Well over $5000 and if Joe is right, other repairs might be required. I would be looking for $10,000 off the price.