Children are told to mind their Ps and Qs when they go visiting. They must be on their best behavior. They have to be able to speak like well-educated young people. They have to know P from Q. Well, so do economists, actually.
Opinion seems to be divided on what the Ps and Qs in question actually stand for. As for economists, there can be no difference of opinion about our Ps and Qs.
Where the economist is concerned, P stands for price and Q stands for quantity. Multiply Q by P and you get V, for value.