See if you can push a small screwdriver through the rust. If you can it will need a new wing. Poke at the scuttle panel with the same screwdriver. If it goes through a new scuttle panel end is needed.
AS for price, a genuine wing is c.£80, scuttle end would be c. £15. The screen has to come out to do a scuttle end. time is in the region of 4 hours for cutting, welding and grinding back (assuming no more rust is found in the inner wing or front panel). The allow for re-painting wing & scuttle. Say around £275 for the whole job if no other rust is found.