Back in the day a penny would buy a little piece of candy that now you have to pay "fifty cents" or more for.
A penny is one cent. When someone would offer an opinion, instead of saying as today "IMHO", he would say "my two cents worth". Related was "a penny for your thoughts" when with someone staring off into space.
After all, our own thoughts are at least twice as valuable as others', right? I know mine are, IMNSHO.