I think you guys are confusing the costs of 'boat ownership' with 'living aboard.' In that one could live aboard a hull docked at a marina and never go anywhere, such a boat wouldn't need an engine, sails, or even a mast, and would be so cheap to buy that depreciation wouldn't be a factor. (You might need to put some $ in tho, for example upgrading from an icebox to a refrig system, etc). OTOH, if he's buying a boat to enjoy sailing and investing in those systems anyway, the *incremental* cost of moving aboard and giving up the house is minimal -- maybe just the liveaboard utility fees at a marina -- instead of the cost of rent plus marina slip. Either of those two scenarios represents cheap 'living aboard.' The place where the best you do is break even, is get into sailing as a hobby for the first time, AND moving aboard.
BTW, Jeff H - speaking of living aboard cheaply, where are those $100/month boat slips you mentioned in your post? We can't find a mooring for less than that!