Disk Brake Rotor Machining

Hey, I'm currently looking at bying a bike that looks like it's in fantastic condition but the seller assures me that one of the front rotors needs to be machined.
I'm guessing the damage would have to be from a drop or something ??
Does anyone know that the cost of having a disc machined would be ?? and how easy it is to get the rotor off the wheel or do you just take the whole wheel in ?
Also if the bike is in this condition should it be ok to take for a test ride when I go to have a look next week ?
Cheers
Dave.
I'm guessing the damage would have to be from a drop or something ??
Does anyone know that the cost of having a disc machined would be ?? and how easy it is to get the rotor off the wheel or do you just take the whole wheel in ?
Also if the bike is in this condition should it be ok to take for a test ride when I go to have a look next week ?
Cheers
Dave.