Because elsewhere "a cheap pen" is defined by you as a pencil, a cheap pen must be accepted also. Be consistent for pedagogy
Actually قلم رصاص (pencil) is never translated as "cheap pen" Pencil (قلم رصاص: qalam rassáss) means literally (lead pen). Not sure why it is called so in Arabic (since it is graphite and not lead that does the job) but maybe this is in reference to its color (Gray/Grey in Arabic is sometimes called Ramadiy رمادي meaning "ash color" or Rassássiy رصاصي meaning "color of lead"). But "cheap pen" would be قلم رخيص and it means exactly that, not "pencil".