This is an interesting question.
One factor which is not entirely intuitive is the tax efficiency of different spending levels.
Because of some quirks in our tax code it is possible to pay no taxes if you have a low level of earned income and spend less than 70 or $80,000 a year.
This means that for every dollar of spending that you wish to do above this level, you need more than a dollar of additional income.
This concept was best covered in this wonderful blog post by go Curry cracker.
http://www.gocurrycracker.com/never-pay-taxes-again/Personally I found this argument very persuasive, and it's convinced me that $2 million is the upper bound for "my number."
Another point I would make is that there is a significant cost to working.
Even though I have a lot of money going out every month, many of these expenses will cease to exist once I hit my number. (mortgage, disability insurance, life insurance, car expenses, professional society memberships…)
And the final point is that many assumed costs are quite fungible.
For me a big one was travel. I used to spend more than $15,000 a year on travel for my family.
By playing the miles game (travel hacking) I'm sure I spend a lot less than $1000 a year on travel now despite traveling to more places, further away.
And to end on a philosophical note, beyond the economic considerations, I've been surprised at how much happiness there is to be gained from simplification.
For me this lifestyle is like eating a single perfectly crafted piece of sushi, as opposed to multiple plates at an all-you-can-eat buffet.