Sorry but to be blunt, if someone can't afford the tips they shouldn't book the cruise. The cruise staff are really lowly paid - ca. $50-$100 per month isn't uncommon; the simple truth is that they rely upon the tips.
It is certainly at each person's discretion whether they book the cruise in the first place. If they cannot afford it due to their economic situation, then they shouldn't book it.
Once on the cruise I am all in favour of economising - not doing shore excursions, or arranging one's own, less expensive excursions, or skipping spa treatments etc, but one shouldn't be looking to economise on tips
unless in the exceptional instance one doesn't receive good service, which is, I believe, highly unlikely on a
Disney cruise ship.