Granted, "Thanks a mint" is an old timey phrase from when people said quaint things like 23 skidoo. But search engines do return instances of people still using the phrase, and the Google question completion offers it as an option as I start to write the phrase. In other words, it's not my imagination that it is a phrase known by others, yet I cannot find its definition in any online dictionary despite my best efforts via Google, Bing, and Dogpile searches.
From the comments I've seen thus far, the apparent explanation is that its usage is too infrequent to merit inclusion in even a crowd-sourced slang dictionary like urbandictionary.com. Is this correct? If yes, are there many phrases that fall into this category? Is Google ngrams the ultimate authority for determining whether a word or phrase merits a dictionary entry in the eyes of this audience? If "Thanks a mint" had returned the same Google ngrams result as "Thanks a bunch," then would my bewilderment suddenly become valid in the eyes of this audience?