Ahh the joys of silver paint.
Get a pro to do it. Even most pro's cringe at silver.
Don't try and touch it up, will only make it look worse. Especially if your asking the questions you are.
But to give you a guide on why it will cost up to $750 for a proper job:
Remove bumper (maybe)
Sand the scratches away
Clean bumper
Apply plastic prep
Apply adhesion aid
Prime
Sand back primer until smooth (light sand on surrounding area)
Clean bumper
Paint bumper blending (fading the paint) into the old paint so you cant see the edge.
Clear coat the bumper
Replace bumper
Polish bumper.
Plus things like mixing the paint, colour matching, masking up the car (if the bumper isn't removed)
2013 Ford Focus ST - Performance Blue, Cobb AP
Gone 2006 3 MPS Sports - True Red, Koni FSD shocks, in dash OEM look Ebay GPS/DVD, Hypertech, RX8 Rims