"What is the one item you often forget to bring, and later on wish you did?"
Answer: any item that I looked at in my gear closet and thought "I've never needed that on any other hike so why carry it on this one?" Guarenteed - the Trail Gods are watching and busily planning their next not-so-nice surprise for me several days from the trailhead. Try carrying your pack after having stepped on and broken one half of your hip belt buckle. A spare buckle is 1.5 to 2 ounces. (my hip belt doesn't have enough extra length to tie in a knot).
It is far easier to forget something you DID bring along, but then did not repack before you left your last campsite. It's usually the little things that get you - eyeglasses, socks or bandanas left drying on a bush, the spoon you set down on the log at dinner time, etc. I have found a variety of items left in campsites; very expensive water filters, hiking poles, hats, assorted clothing. Wise are they who, having packed up and are now ready and anxious to go, turn around and carefully scan the campsite for anything that may have decided to remain behind. I spent an hour crawling around one site before I finally found my reading glasses in the dust.
It's your money, spend it however you like.