On the way to school my daughter heard someone on the radio offer their 2 cents on an issue and asked what they meant. I patiently explained in a clear manner (like I always do) that it was when you offer your unsolicited opinion and, opinions being what there are, admitting that it isn't worth much. I said that the phrase has been around a long time.
I then realized that, wow, 2 cents used to be able to be able buy something but not anymore (I remember getting gumballs for $0.01). Given inflation, unsolicited opinions are worth a whole lot less today than they used to be worth.