The card from clear only supports 4G, so when you leave 4G coverage you have no internet. The Sprint price plan includes 5GB of 3G, hence the additional cost. More than likely Sprint will eventually offer 4G-only devices and plans which will be cheaper than the current 4G/3G offering.
reply to dib22 because sprint has the ability to do 3G/4G and clear doesnt, so when you travel your card will work with you. clears service ONLY works where you have 4G service