There is no charge for these as long as you get off at a city on the direct route and you complete you trip within the total time allotted by your ticket. No reservations are accepted on bus trips. To find cheap flights look at the airlines web pages every Wednesday at 12 am when they post their sales. You can also be placed on their mailing lists so you receive notification of flight sales. United Airlines offers a service called e-fares which finds the cheapest flights available on any airline to your destination.
Travel agents are also a great tool for finding cheap flights.
In general, hotels are the most expensive places to stay in North America.