Reddit reviews Architecture Without Architects: A Short Introduction to Non-Pedigreed Architecture

I've read "Dwellings" by Paul Oliver. It's quite good. It only speaks to housing, but it is geographically diverse, speaks to a variety of issues including culture (spatial relations, ornamentation), construction techniques, environmental suitability, and informal settlements, for both urban cultures and very remote settlements. Amazon lists it at just under $30 (at Amazon.ca, they have 2 copies left, unlike Amazon.com). If cost is a major barrier, you could also consider trying to find it, or others, at a library.

Oliver's book is more of a catalogue. Academically, Bernard Rudofsky's "Architecture Without Architects" is a seminal text on the subject. I haven't read it personally, so I can't make a recommendation, but I will say that I would personally be interested in reading it. I imagine that this book would try to locate vernacular buildings within architectural discourse, and is probably less comprehensive geographically, culturally and historically than Oliver's. And it is almost within your price range.

Post modernism has largely come and gone, however Complexity & Contradiction in Architecture by Robert Venturi is excellent.

Also, Architecture without Architects....

and

Lessons for Students in Architecture

I'd recommend any of the structures in this book: http://www.amazon.com/Architecture-Without-Architects-Introduction-Non-Pedigreed/dp/0826310044