My job just before I started my PhD paid 19,500 British Pounds per annum (that's about 39,000 USD in 2007 dollars, which inflation adjusted would be about 44,150 USD in 2015 dollars). My graduate student stipend was 27,000 USD a year. And yet, I probably lived more comfortably, and certainly more independently in graduate school than I did back in London on that nominally higher salary (cost of living in the most expensive city on earth and all). My current salary still has not quite reached the inflation adjusted dollar value of my previous salary. Yet, day to day, in a two income household with no dependents in an inexpensive part of the country, I live well enough. Certainly it would be disingenuous of me to claim poverty when you look at some of the things I have spent money on in the past few months. As I write this, I should also add that I am pretty much in the Rolls-Royce of postdoc scenarios financially. I'm funded out of my PI's R01, with full staff benefits. Also, as result of having done my undergrad in Europe, I have almost no student debt. And, as a result of my mother's generosity, I have no structural debt at all and never have had. It would not take much (a kid, a more expensive part of the country, a large amount of debt to service) to shift me into a position that would be financially much harder.
Which leads me to my main point: I could probably live quite comfortably for a long time on this salary. I could build up a retirement, maybe make some investments. All good. But that is not a scenario I can contemplate. Because my employment is of fixed term, and I am always facing the knowledge of the imminent cessation of income. It is this combination of limited, low-ish income and limited term contracts that is the particular financial difficulty of the pre-faculty (and let's be honest, pre tenure) phase of academia.
I saved throughout graduate school, a fair amount. But those savings were devoured by the process of having to finish my dissertation after my funding ran out. Even had I secured a job, in all likelihood moving costs alone would have demolished my savings. And postdocs employers do not usually pay moving fees (for my current postdoc I was lucky: my PI paid for my flight from the UK, because she is wonderful). Further, had it not been for the opportunity to live back at home (which comes with its own personal costs), I'm not sure what I would have done financially. Certainly, I would have given up looking for a postdoc sooner than I did.
The problem is that grad school stipends and postdoc salaries are, quite simply, too low to put together a nest egg sufficiently large to insure against the periodic cessation of income, or to weather any prolonged period of unemployment. Thus, for most of us, when our graduate stipends or our postdoc grant salaries are coming to an end, there is significant pressure to simply secure a source of income as soon as possible.
Which brings me to my next point: there's a lot of discussion about people taking postdocs who aren't committed to science. Or talking about how you should only do a postdoc if you really want to. It's bollocks. The main reason, when you reach the end of your phd and are staring at your paltry savings to have a postdoc lined up is so you can get paid. It pays more than grad school, and it buys you time.
Yes, career transitioning would be better for many. But career transition is a hedge, especially in this job market. It requires that you invest either time, or money, both things that are in short supply at the end of your PhD.
So if you need a postdoc to keep paying the bills, take it, it's a honorable thing to do for you. And try to build a nest egg big enough so that you can get out of there if that's what you decide to do.
As a postcript: this problem, the problem of the financial costs of fixed term entry level jobs, is one that is well documented in one of my home countries, France, where all entry level jobs are fixed term CDD and people coast from CDD to CDD until they land a CDI. This has huge financial consequences on young people, from the impossibility of getting a mortgage, to financial dependency on parents as garantors of all loans, to requirements to move around France, to inability to build up savings or retirement benefits. The precarity of fixed term employment is real.