The cost is going to depend on the type of Insurance Plan you have. Sometimes the cost of X-rays for Orthodontic treatment is not covered at all so you need to review your plan benefits. If you do not have Insurance then the cost of any X-rays would depend on what the dentist is charging. Dentist to dentist cost varies and would be hard to say what one charges and why. That would be something you would need to shop out.
If you are looking for a discount plan to help off set the cost of your orthodontic treatment most discount plans will save you about 20% for that type of service. You do need to go to a dentist in their network. There is no out side network so if you are trying to stay with a current dentist make sure they are in the plan before buying one.