It's a new one on me, too, Ensiform. But spendy is known to the Oxford Dictionary of English, which gives two meanings: expensive, and extravagant. The Oxford says it is chiefly US, but some sources are more specific, and say it's chiefly Pacific North-West.
An early example with the expensive meaning was in a 1992 book, Runabout Renovation, by Jim Anderson: "Aluminum definitely makes for a first-class live well, but it does get a little spendy, and you may opt for wood."
Examples of spendier are a little harder to track down, because it seems to be a common German word. However, I think it would be appropriate to add spendy, spendier and spendiest as rare words.
