#7 You dont need much money if you know how to do it. My granny is at home just a few weeks each year (important things like x.mas, birthday) the rest of the year the is travelling around the globe. And i can tell you..she is not even close to be rich. Now she got time..yes. But a travel around the world dont has to be expensive and you dont stay weeks in every country. As #5 said..if you dont need 5 star hotelst and first class flight you can see very much with less than 10k and a month vacation. If i save my money for that goal i could do it in about 7 years..and my income is pretty low.

#12 as #14 said..some spare money and enough vacation =) Lets eat grandpa / Lets eat, grandpa
But if you got an "average" job, set your goal to do that a decade in future. Save money..safe overtime and vacation days (if possible) to take 1-2 month off work and plan well..then its possible even for average to low income people. Disciple to save money for a decade for just one goal is the hard part (since you save money for other things too). Or you create a simple blue-white my-space alike website, get a hype and become billionaire =)
Planning is everything..plus a bit of luck