I think $50 is a fine gift, $100 if it is a close relative or friend. $50 is still a good amount of money for a 17 or 18 year old, remember!
Great suggestion for a gift card for Container Store. Other good gift card options for new college students -- Amazon or B&N for text books; Bed, Bath and Beyond for sheet sets and storage cubes; or Target for everything from toiletries to dorm room snacks. Gas station cards and gift cards for Starbucks or Subway will also be a hit, but maybe aren't as memorable as a card towards something more tangible like text books.
For something a little more personal or creative, you could google their college and find out the going rate for the fitness center and pay for that, either directly or putting the balance for a semester or the year (probably $50/term at many schools) on a check and gifting it along with a gym tee from the college.