I just called carmax at white march and the young told me i had to make appointment to come in to get a price on my back brakes. I don't understand why I was not able to get a price over the phone

The cost will depend on what needs to be repaired. Is it just brake pads? Are the rotors damaged? Calipers? Maybe it just needs fluid. Without looking at the vehicle they won't know what is needed. But I'll tell you this... The longer you wait to make the necessary repair, the greater the risk you run of making matters worse, and the more dangerous it can be. Don't wait until your pads are worn so thin that the rotors get damaged. (NOTE: I am not a mechanic, so I apologize if my terminology is incorrect. But, I've learned the hard way to get them fixed sooner than later.)

It is bcuz they need to look at your car at the carmax place to determine whether or not there are other things wrong associated with the brakes issue.

Agreed. Do you need pads only or pads and rotors? Do you want the same type of pads that you have now or different type? To many questions that need to be answered by looking at the brakes, not just guessing over the phone.