Well, I don't think the OP's FLGS is an isolated occurence at all.
A couple of weeks ago huge swaths of 4E supplements went into the $10 a piece area of major retailers, like
this one (make sure to check the variety of 4E supplements that have taken a nosedive in terms of pricing by also checking the same seller - BookOutlet - on abebooks and the amazon marketplace area).
I'm sorry to say, but there are exactly two circumstances where we saw a similar pricing drop in WotC' recent history with D&D:
1. Eberron supplements. Just didn't catch on. By 2007 you could get 10 of these for $5 a piece on abebooks - from a high number of retailers.
2. The end of 3.5. Mid-late 2007, lots of stuff went into the $5-$10 price range. Like, new stuff, aggressively pushed off the shelves. I should know, as I'm one happy voracious D&D buyer of that period.
You know, for all the proclamations of WotC to the contrary, it's pretty undeniable that the retailers do behave as if product is being phased out and overstock cleared as if there was no need to sell it competitively.
The interesting question is: are these retailers acting on insider information that WotC wouldn't care to share with the customers, or are they acting on their own instinct?