You will probably get a better response on body and sheet metal questions in the Body Shop section of this site, just for future reference.
Any time a repair is noticable, it will lower the value of a car in my opinion. That is not to say putting a new 1/4 panel on will lower the value, however.
If it were me, I would probably opt for a full 1/4 replacement from a quality repair shop. It is really hard to say what the cost will be but you can help yourself by removing the rear bumper, the inner 1/4 trim panel, rear seat, sail panel trim, etc.
Try checking with some Camaro clubs in your area for some recommendations on shops and be prepared to supply the panel, if necessary. Ground Up is the sponsor of this site and they carry the full 1/4 panel you need.