Where do you live?
I live in the Boston-Washington DC metro area so I am a 4-8 hour bus ride from every school I interviewed at (17)
If you don't know what the Bos-DC metro is click this link:
http://en.m.wikipedia.org/wiki/Northeast_megalopolis
NY State, Pennsylvania, Virginia, DC, New England, Ohio, North Carolina.
The schools in the northeast met my needs (state schools, Ivies, hometown etc.)
I only needed to fly once but flying round trip in the northeast is cheap (usually under 200). I also worked really hard to get hosted.
I spent about $1,000 on travel.
If you don't live in the Boston-Washington DC metro area than things will be much more expensive.
I also decided to apply to only schools in the Bos-DC area or close to it which includes most of the top 20, all of the NY, Mass, Conn, Virginia, Jersey state schools, all of the Ivies, schools like Virginia Tech, Temple, Duke etc.
You can save money too because schools are close.
For example, you can interview at Sinai, Downstate, Yale, Temple, Rutgers, Stony Brook, and Penn virtually back to back because they are all connected to the NY metro hub (Metro North, MJ Transit, express service to Philly from NYC)
Megabus is a good option. From DC to can pretty much get anywhere in the east coast.
If you farther south than virginia and/or farther west than Ohio it will be much harder/expensive to apply if you are not only applying in state because you will be require to fly.