I cannot give a estimate myself without more information. You should take it to a few local paint shops and dealerships to see what they would charge. That way you know you are getting a good deal. Always ask to see an example of their work beforehand though.
Depends on how bad the scratch is. Most auto body shops will want to respray the panel, which will cost about $500. But you might as well try a "scratch fix" product that you can pick up at a local automotive store, which may possibly buff out the scratch without a problem. Cost: about $10.
depending on the scratch it could be as simple as a quick feather rub & a light spray , a fill & paint or the worst case scenario would be work on the whole panel , most workshops factor about $ 200 per panel , so it could be between $ 200 - $500 depending on the filling work needed
A simple scratch is normally not to costly. It can be often just buffed out. If you have a deeper scratch then it may need to be touchup painted. This will determine the cost of you repair. Talk with some body shops and go form there.