Its relatively cheap to travel abroad, especially if you're willing to live simply while you're there. If you plan significantly far in advance, you can usually find airfare for $500-750 round trip, or cheaper, depending on where you want to go. Then if you plan it out, you can usually stay wherever you are for <$100 a day.
I'm not sure about where you are, but even someone in the US, working full time at McDonalds, making $6 an hour, could afford a week long trip every year by putting away 10% of each paycheck. $1200 is (roughly) enough for a week long trip to most of the non-tourist packed places, and you can save that up by putting away a little more than $20 a week. Just making lunch at home rather than eating out (or buying a pre-made lunch) can save you almost that much. Its just a matter (like he said) of priorities. That isn't to say that some people can't afford it. My example assumed someone with no family, medical problems, or other large expenses, but it was also a fairly low end of the pay scale (for the US).
$100 a day!? That's not living simply - that's staying in hotels and eating in restaurants. I'm an active CouchSurfing host and I've met many travellers who get by on $100 per month. And that's in Europe. You can stretch that out for 3 months in lots of parts of Africa or SE Asia.
CouchSurfing and equivalents can drastically cut your accommodation expenses. Hitch-hiking or cycling give you free travel. Even travelling by bus can be remarkably cheap in most countries if you go the slow way (which also lets you see much more of the country that just the capital) - you can often go 50-100km for a couple of euros. Your primary cost is food, and if you've learned to cook even a few basic meals, you can easily get by on a couple of euros a day.
