it's funny. I got into an argument at work with a girl who insisted you had to be a millionaire to circumnavigate. Anything I tried to say otherwise was met with a "********!" statement from her.. or the famous "Lies!"
I will admit, except for rent and paying for my Disco, I live pretty cheaply. Factor in Electricity, Gas, Cable, and water.. and I bet I could live on a few hundred a month as long as I could get cheap places to drop the hook or pick up a mooring
It all depends on what you call a millionaire but it takes money to do that unless it is a non stop circumnavigation: Many thousands of dollars, more than the price of some old cruising boats around.
I do extensive cruising (about 4 months a year) and live in a boat. Living on two pensions (me and my wife) have cut our income a lot but i figured out that I could not have money and time and I choose time but with the kids yet finishing university and having that boat and house to maintain I cruise on a tight budget but you need a budget for cruising otherwise you are surviving on a boat.
Capta had already talked about maintenance costs and I would say that, with the winter parking of the boat would amount at least for about 4000 euros. Only that would amount be near the 500 USD you are talking about.
Regarding cruising, even staying out of marinas, with only rare visits to some inexpensive ports and eating a few sometimes out, spending very few diesel and sailing almost all the time I guess that I spend (me and my wife) between 1000 and 1500 euros a month.
Off course, if we stayed a lot of time on one of those rare places where the shelter is perfect and the life is good, we could make it with less, but that would not be cruising. Cruising for me is to be on the move, discovery new places and enjoying sailing and that is a lot more expensive that staying or living in the boat in one or two places.