We've explained it before, but it's worth another post.
Robb and I are not rich. We both work full time at our jobs which pays the bills. When we travel, we are very budget conscious and look for deals on flights, hotels, and car rentals. We often split car rental and hotels with friends to help bring costs down even further. Also when we travel we cram a lot in to a 3 day or 4 day weekend. By taking redeyes, travelling over holidays we don't need to take off that much time from work.
We travel with many poor college and high school students, as well as families on budgets. With some careful research and planning you can do these trips within most peoples budgets.