There is no "one size fits all" when it comes to answering the question: "how much money will you need to comfortably retire on"? It is entirely dependent on your retirement goals, desired lifestyle and how flexible you are.
The most common wisdom that you will find on the internet will tell you that the number is 70% of your pre-retirement income. That number will of course vary depending on your expenses which in turn will depend on where you are living. The good news is that - if you are flexible - there are different retirement options for almost every budget if you are willing to move where the costs can accomodate your income. For example an article in MoneyWise covers 20 different countries where you can retire on a lot less than you may think is possible.
Countries such as Portugal, Uruguay, France, Costa Rica, Portugal, Thailand are covered in the article where yes, appartment rentals can be found ranging from $350 to $750/month. If you have a retirement income of $2500-$3000/month for example, you may find that one of these countries could serve up a workable and enjoyable retirement.
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