Swords do not consume neither Magic nor Rupees when you edit a current sword or make an all-new custom sword. I want to make a variety of swords for my quest, one of which always shoots sword beams & has high power but it consumes magic every time you swipe it. I tested this out & it does not consume any magic. (I am using beta 273)
Swords & Magic or Rupees
Asking for it is what you have just done!