Polishing to remove stains on dental bonding? Is this an option? What are the costs?

If your dentist splinted your central incisors together in order to achieve an esthetic result and not for any other reason (such as stability or orthodontic retention etc..) then have a dentist unsplint the teeth and perform a normal "diastema closure" using filling material. It will look better than splinted teeth, it will be easier to clean with floss, and it will not stain. Also way less expensive than veneers or crowns. 
If you want to keep them splinted then the cost would probably be included in your 6 month hygiene appt. She/he can probably remove the stain with an ultrasonic instrument during your cleaning. I don't have a fee for this to use as an example because I would never splint teeth just to close a gap.