An apocalypse is a near impossible thing to plan for. Knowing when and how one will strike is as unlikely as predicting earthquakes and weather patterns. Therefore dumping valuable resources into preparing for such an uncertain catastrophic event is as foolish as the Y2K hysteria, or the various doomsday cults scattered about the news in recent decades. Over time, our fiery anticipation of cataclysmic disaster does far more damage to our psyche than would the apocalypse itself. If being prepared is considered absolutely necessary, however, it would be wise to utilize creativity and caution to keep what measures you take from infringing negatively on a healthy budget.
Several steps will ensure you are successful in accomplishing this. Consider foremost, if you are fortunate enough to survive into this post-Christian world, what factors will be essential to your continued existence and pursuit of happiness. Laughter is supposed to drastically reduce stress levels and even potentially boost intelligence, both critical attributes contributing to sound decision making, which is what will decide, in part, your quality of life and how long it lasts.
You will want to find things you will use anyway, such as tools, canned goods, books on weapon making, edible plants, and jokes, perhaps. If you cannot afford to buy the books, stake out your local library and devise a scheme in which it will be easy to break in. Make a list of the most valuable books to take and their location. Again, local edible plants and making weapons should be the main topics of interest.
Next, it is unlikely that there will be any means of communication once this apocalypse occurs. You will need to make plans with those who you care to meet up with after the dust settles. Plan a sort of language of smoke signals or tree markers so that you can find each other. Make sure all of these notes are written down and that every involved individual has a copy. This will cost no more than some lead and a scrap of paper, but can be one of the most valuable tools when the event strikes.
Finally, you will want to build an underground safe house. You can store all food and ammunition and such there while you roam about in search of other essentials. You will perhaps try to make use of old tires or recycled scraps, which will be able to go toward the construction of a potential shelter. This all costs very little. The largest cost is in energy. Best of luck.