CPT® Walk-In Code

I am interested in billing a walk in code at a special rate of $50.00 as many walk-ins are doing across the country. Many are billing 99213 as the code. How can I bill without lowering the charge on all of my office 99213 codes? I know the charge for a code has to be the same for everyone, but what is that based on, per tax ID? per location code? Not sure how this is being done. House codes may not be covered by insurance companies. Can you shed any light on this?