don't forget that you can also resell your contract. That's one thing a lot of these simple formulas don't take into account.
Yes, there's a chance that the whole market tanks and the resale market will suck, but what is the likelihood that occurs at the same time you're considering selling?
For example, if an initial purchase price of $10,000 saves you $400 a year in accommodations. The break even isn't 10,000/400. You really have to look at it as an investment in which you spend $10,000 in year 0, and then each year it returns $400 of savings...and then in year 10 or 15 or whatever year you think you'll sell it, you get your $10,000 investment back. You could also discount the resale at the end if you want to be conservative. And if you really want to get complicated, you can do a discount factor taking into account opportunity costs (e.g., time value of money).