Given the fact that Helium is now readily available (as compared to times when Zeppelins were around), why isn't Lighter Than Air Aviation used for cheap travel?
There are many factors that have kept Lighter Than Air Aviation out of the mainstay,
Speed: Airships are not exactly the most efficient means of moving through the air. As such their speed is far more limited than their heaver than air counterparts. For what its worth the main advantage of air travel in the modern world is speed. Its better to fly to Europe in 8 hours from New York than take a 3 week boat trip.
Capacity: The largest airship ever made was the Hindenburg class of airships. Coming in twice as tall and 3 times longer than a modern 747 and in its biggest configuration only able to cary 73 passengers compared to 550 for the largest configuration of the smallest 747 built. Granted the Hindenburg was not laid out like a modern airliner (it was more like a cruise ship) considering the slow speeds it would need to be that way even today.
Even with Helium being easier to come buy, its just not efficient to fly such a large craft for so few people.