What is the cheapest way to travel?

Your own two feet. Course you won't get too far but hell man...it's sure cheap.

by plane if you are going overseas

There are all kinds of ways to travel for free. People hire others to drive their cars cross country. Some people will buy your ticket if you escort a child or an elder.

Walking I ssuppose

Depends on how much you value your time.

On foot.
Scooter.
Bicycle.
Skateboard.

Train jumping.

Greyhound bus

Imagination!

astral travel

Rich friend.

Shanks's pony.

hitch

Walk.

Bike