I have heard the phrase "75-cent word" used to describe an elaborate,
lesser-used word that is used in place of a simpler, more common one.
For instance, "clandestine" could be a 75-cent word for "secret." 
Could you please find the origin of this phrase?

I've often heard fancy words called "2 dollar words." 

Maybe inflation is running the price up. ;-)

Gosh, Pink, words must be worth a lot more in Tulsa than here in
California!  I still hear the expression "two-bit word" (two bits, of
course, being 25 cents).  Come to think of it, I've heard the
expression "forty-dollar word," too--but that was back East, where
words are known to be pricier.

Archae0pteryx

If your handle is archae0pteryx you'd obviously prefer 40 dollar words.
I almost typed four letter words, but they come cheap.