I agree with those below the very FIRST thing Id do is remove, clean n wire brush, then reattach each n every battery n starter n ground cable. Also on some cheap cables they can go bad underneath the terminal crimp where you cant see.
In cases where the voltage dont drop much when trying to start, it could be a cable or connection or starter switch or starter solenoid problem, although sure still a bad starter like worn brushes. In cases where the voltage drops drastically then I suspect the starter more like bad bushings or dragging or shorted armature.
Is there a way to safely by pass jump around a starter switch or solenoid with heavy jumper cables to see if thats the problem??? if so i.e. you can get good access to starter post without accidental shorting, give that a try.
Sure starters go bad, but more often its a cable or switch or solenoid or battery problem
John T John T