My camera bag contains some costly pieces of equipment and, stating the obvious, it would be impossible to do my photography without them. Equally though, there are plenty of inexpensive items in there and the price I paid for them is completely out of proportion to how important they are to me when I’m out.
For instance, I would no more want to set off without my tea flask than my camera and lenses; nor leave home without the heavy groundsheet that was a bargain from the Poundshop and protects my bag when I lay it down on sandy beaches or the woodland floor. And I've lost count of the times I've been grateful for my head torch.
Recently I acquired another cheap piece of kit that has since become indispensable. For £10 I bought a completely waterproof dry sac made by a company called Exped and their extra extra large version fits comfortably over my longest lens, even with the neutral density filters attached, and it weighs next to nothing. There are now times when the dry sac enables me to make photographs on the very edge of the weather that would have been extremely difficult before, especially in upland landscape and at the coast.
Rather than putting the camera and lens back in the bag when a heavy shower arrives, with the dry sac my gear remains on the tripod, safely covered up and is still set up and ready to go the moment the rain stops and dramatic shower light hopefully returns. Good pictures were missed previously because in the time it took to get everything back out of the bag and the photograph recomposed, the best light was long gone. So the £10 dry sac has been money very well spent.
One downside to be aware of is that when you use it on a really windy day then you must keep a constant hold of your tripod, as the sac, while waterproof, is like a big wind sock and there’s a risk the tripod could be blown over in a strong gust if you leave it unattended.