Commas and Partial Quotations

What are the rules for comma placement when it comes to partial quotes?

My intuition is that a comma should be placed outside of the quotations marks if the comma is related to the overall structure of the sentence. However, the various sources I have looked to (APA, MLA, and others) have stressed that commas (and periods) should always be placed within quotation marks.

For purposes of illustration, would the second comma go within or outside of the quotation marks below?

As Andrew argues, peanut prices “can be ridiculously overpriced", leaving many Americans to resort to jelly sandwiches.

or

As Andrew argues, peanut prices “can be ridiculously overpriced," leaving many Americans to resort to jelly sandwiches.

An explanation of the general rule would be lovely as well.

Answer

As it was explained to me many years ago (in Clinton, Tennessee, USA) by a linotype operator who was old enough to have set type by hand, "[Punctuation] goes inside the quotes because editors are too dumb to know where it goes and, if we have to have to talk about it every time, we'd never get the paper out."