When people ask me what’s on my mind, they’ll say “a penny for your thoughts.”
When I take the initiative to tell them what’s on my mind, it’s putting in my “two cents.”
We’re commanded not to overcharge or undercharge by more than a sixth (see the fourth perek of Bava Metzi’a). Isn’t this clearly a violation of that, in that I’m giving something worth two cents when he only is offering one?
This assumes that all amounts discussed are at least a shava perutah.