I have often tried to 'monetize' the value of a car - but I find it comes down to this:
How much $/month is it worth for you to have a running car for a month ? and what does that car *need* to have ? How much extra$ are you willing to pay for new-car reliability ?

All of my cars were purchased used, thats a risk. but its also a risk I can take (If I'm late to work, I wont get fired or make less money that week, we have 2 vans so if one breaks, we can still go somewhere as a family, not everyone gets this kinda deal)
If my paycheck was partially dependent on looking professional, arriving on time, responding to callouts, and maybe carrying extra people (I know a lot of industrial salespeople like this) then a used car would simply not be considered. its a new minivan or a new job. In this case, I'd happily pay lease payments on a new van because now my time is worth that.

If you could put a dollar figure on your time, and how much you are willing to spend to mitigate the risk of a broken car: this decision would be much easier.

I cannot. (because among other things - time with my children is very expensive...)

-John

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2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

Husband, Father, sometimes friend =)