The really unfair thing about being poor is that things cost more. Not just in relative terms - when my income halved, my mortgage went from being 20% of my outgoings to 40% - but in absolute, dollars and cents terms. The less money you have, the more you have to pay for the same things.
You can't buy in bulk
If you only have enough coming in to cover the week's essentials, or what you need right now, you can't take advantage of any of the money-saving offers or buy in bulk and stock up. A 250g bag of rice for $4 is obviously not as good as a 1kg bag for $7.50, but if you don't have the extra $3.50 right now, you don't have that choice. You're now paying $16/kg for rice - over twice as much for the same thing as someone with a little more flexibility in their income.
The same's true of just about anything else you buy. Small quantities cost more, whether you're talking about food, washing powder, or season tickets. When you're managing your finances day to day, your cost of living goes up, right when you need to be saving money.
You can't take preventive action
Everyone knows that maintenance is cheaper than repair, but if you can't afford regular maintenance, sooner or later, you're going to have a major incident that will inevitably work out more expensive. It's the same with insurance. In the long term it saves you money, but when money's tight, it's one of the first things to go. And of course, when one of those repair bills comes in, it's catastrophic to your finances. What would have been a quick $50 maintenance job if you'd had some spare cash turns into a $500 disaster.
Finance costs you more
If you have money, you don't pay bank fees. The banks will fall over themselves to offer you free banking. When you're not earning as much, though, then you suddenly find that every month, the bank starts charging you just to keep your account open. And if you accidentally slip up and go overdrawn - often thanks to those damn service charges - you'll end up with hundreds of dollars in surcharges, right when you can least afford them.
The cost of credit is also often tied to your income. The less you earn, the more expensive it is to borrow money - not just cash, but car payments, mortgages, and so on. If you don't have the cash for a $400 fridge when you need one, the actual cost over a few years could work out to way over $1000. If you were earning more, but still wanted credit you might only end up paying $600 for the same fridge.
You also don't get the opportunity to pay bills in instalments when your income isn't great. Not only does this mean you have to find the entire sum in one go - a huge problem when you're living week to week - but many places give you a discount if you spread your payment. It's only poor people who pay full price.
You can't buy money-saving devices
Technology is full of gadgets that can reduce your outgoings. They involve a bit of up-front cost, but your monthly bills will go down, and over a year or two, you'll see big savings. These can be simple things like heat-reducing window coatings, low-flow shower heads, or energy-saving refrigerators. Looking at larger items, a new a/c system, a new boiler, or installing solar panels will have a big payback over a few years, as long as you have a few grand spare to install them.
When things are tough, it really hurts to realize that our economic system is designed so that you're paying over the odds for almost everything you buy. Not only do you have less money, but what you do have doesn't go as far. It's a cruel double whammy.