If you are broke after buying an iPhone, do yourself a favor and leave it in the box until you can afford a proper cover.
The quickie DIY cover demonstrated in the video below is neither smart nor attractive, but it does demonstrate just how much thought people put into different ways to live with their devices.
The balloon trick, while probably keeping part of the phone protected from rainy weather, renders the camera useless. On the plus side, if you drop your phone from a great distance, the balloon should keep your broken bits together.