I would also add a few more things... which are in my bag: 16) Rubber bands - you can use a rubber band to strap down a flash diffuser, snug your strap to your shoulder or even hang a backdrop (provided its lightweight, of course). 17) Ziplock bags - I always keep a gallon size and a couple quart size ziplocks in my camera bag. They are great for keep things dry and/or sand-free and provide excellent temporary cover if the weather should change, but you need to keep shooting. 18) Batteries - This one has a high DUH factor, but batteries should not be forgotten. I carry at least four AA's and two AAA's with me, at all times. That way, I can always change my flash and trigger/receiver batteries, as needed. Although your camera battery might not fit the "under $25" part of the list, it's a good idea to purchase a second battery for your camera. I keep two batteries and even a 2nd charger handy... to minimize points of failure for any shoot I conduct.
What's are the must have items - under $25 - in your bag?