My SO and I have lived/cruised on a couple boats in the past 3 years. If you look through the posts on my profile, you'll probably find other relevant ERE sailing threads. We also have pictures of our small (25' and 35' boats) and what we've done with them on http://peanutbutterdiet.blogspot.com. There is also a good, though small forum called SailFar for simple sailing: http://sailfar.net/
If you want some numbers, we sailed from Port Lavaca, TX to the Florida Keys. I kept spreadsheets of all my expenses at that time. I averaged ~$80/wk plus my health insurance and phone (~$435/mo total) We only stayed 2 nights in a marina because we needed shower power for some tools to make a repair. We didn't have any refrigeration or insurance. We had a 25' boat with a 9hp gas outboard that used between 1/3 and a 1/2 a gal per hour. We used propane for cooking and all our electricity was generated with a solar panel. So, yes, you can go cruising for <$500/mo. And go out to eat while you're at it.
The other thing to note though, is that we a lot more spending leading up to the trip. The boat cost ~$8000, the engine cost ~$1000, the dinghy cost ~$700, replacement batteries for an EPIRB cost $300+. We bought a toughbook + a GPS and used opensource software instead of a chartplotter. We also bought foul weather gear, chartbooks, courtesy flags, BoatUS tow insurance, etc. During the course of the trip we needed to buy a new shroud ($70), fiberglass, and epoxy. Also, as we kept the boat for less than a year, we skipped out on the cost of all the yearly maintenance jobs. Had we kept her, we would have had to put considerably more work into her to protect her for the long term. Also, as boat parts are expensive enough, I would plan to do all your own work. For a DIYer, that's probably no problem though.
I don't know that I go for all the simple cruising ideas listed though. I think our iceboxes were worthless. The ice melts fast and costs too much. Once I figured out we were spending $50+ a month on ice, I gave up on it. Also, unless you keep ice in it all the time or dry and disinfect it religious after each intermittent batch of ice, it will become a source of hot, dark, wet, moldy nastiness. Living with no fridge is by far the #1 suck of boatliving. Also, considering how expensive paper charts are, I'd say fancy navigating is cheaper. You can put OpenCPN and free digital NOAA charts on a laptop for nothing. Add a GPS dongle and voila. Having paper charts as backups for everything is ideal, but for a long cruise, it's damn expensive. Plus, honestly, in close quarters I accept that my chances of smashing us into something because I can't steer a perfectly accurate course in crappy weather is greater than my chances of crashing into something because all of the four GPS devices we had floating around the boat have all gone south.