The parent's comment point was that it may logically be assumed that you'd be doing such an action once every 1 year vs once every 3 years, for example.
Edit: Upon realizing that you'd save the $50 every time you go / every 30 minutes spent regardless of how often, me (and presumably the parent) are wrong, and I'm going to bed. :)
Whether the device will last 1 year, or last 3 years, doesn't change whether you should spend 30 minutes to save $50. The rich people who changed their answers were still illogical.
Edit: Upon realizing that you'd save the $50 every time you go / every 30 minutes spent regardless of how often, me (and presumably the parent) are wrong, and I'm going to bed. :)