I would format and punctuate the quoted sentence as follows, based on your comment to Ant that the words "What do coffee pickers with burros think about all day?" represent the title of a book:
What is the purpose of human existence, or is it meaningless? is the question asked by Juan Valdez in What Do Coffee Pickers with Burros Think About All Day? (p. 80).
Most style guides prefer that book titles be (1) italicized and (2) rendered in title case (initial caps for every word except articles and short prepositions). They also specify that question marks in the title be included in references to the title; and since you say that the question mark is part of the title, it should go immediately after the word Day and be italicized as well. Acting on these pieces of advice yields my styling of the title as What Do Coffee Pickers with Burros Think About All Day?
The next question is whether to include a question mark after the word meaningless. On that point I think The Chicago Manual of Style, fifteenth edition (2003), is on very solid ground when it recommends including such punctuation:
6.71 Within a sentence. A question mark is used within a sentence at the end of a direct question [cross reference omitted]. If the question does not begin the sentence it need not start with a capital letter [cross reference omitted].
Is it worth the risk? he wondered.
The question, how can the two be reconciled? was on everyone's mind.
Of course, if the wording "What is the purpose of human existence, or is it meaningless[?]" is a direct quotation from page 80 of the book, the question should have quotation marks around it.
And finally there remains the question of what to do with the page reference (to "p. 80"). My own preference would be to put it immediately after the part of the sentence that contains the cited language (or passage). So here that would yield a sentence along these lines:
What is the purpose of human existence, or is it meaningless? is the question asked by Juan Valdez (p. 80) in What Do Coffee Pickers with Burros Think About All Day?
But if you consider its placement at the end of the sentence an aspect of the writer's creative voice, you can certainly leave it there—though it should be followed by a period in that case.
Having said all that, I reiterate deadrat's view that in the world of style decisions—the world you're asking about—rules are not universal. In fact, they're highly subjective and you may or may not be free to choose the ones you find most compatible. From a reader's perspective, stylistic consistency within a book or article is more important than the particular style guidelines an author adopts.