As a rough rule of thumb, you see about 15% of the rust on a car when you inspect it - six times more is hidden, but anything made of sheet steel can be repaired with sheet steel, with time, effort and a MIG welder.
Not a big deal really. But Brian is right - more is surely hidden. That being said, tackle the rust that affects the structural integrity of the car first.
Won't be able to afford a full rust body repair. Attending Uni and working part time has limits sadly.
Will be fixing that exterior rust spot however, in the near future!