Personally, I think it's entirely possible for someone to have bought a genetic granite at a normal price. I also think it's entirely possible to have a snake that has granite traits without it being a specific gene that can be passed on to its offspring. The variability in ball python genetics is crazy like that. The only way to prove an unknown granite is to breed it out and see if it throws true. I don't think it's fair to say someone's snake is definitely a normal just because they got it from an unknown source at a normal price. Nor do I think it's proper to say an unproven snake is definitely a granite just because it has some markers.
Just my two cents. ;)