Ebay auctions (avoid Buy It Now). Like I said, plenty of copies are going for under $90 on there. 5 of the last 6 copies sold in the last few days have gone for less than $90, two were disc only and went for under $80.
You've gone on a few times about these used auctions.. So let me tell YOU a story..
I've bid on this game for the last 2 or so months on ebay. First a about an $80 max bid, and in the past month about a $95 max bid. Guess who just bought one of these copies today? That's right, the guy who hasn't won an auction on the game yet because it's been going for over $100.
What you are seeing "the last couple of days" as you put it is the fact that this first appeared at GameStop.com a few days ago online. The difference between that and today is that now these are available in store.
And most of those auctions still ended up around $80-90 and still have shipping on most of them. I got my copy for $81 not even bothering with coupons. You could get this from .com pretty easy for $60something, and probably in the store for $60something if you have PUR points to redeem or a coupon.
The only indignation here is the tin foil hat conspiracy that this is a reprint. But most signs point to it being a result of the trade-in campaign. Yes it sucks that this is the situation. But a lot of classic Wii games are falling into the high price range. Fire Emblem, SMG2, Pokemon Battle Revolution, obviously Metroid Prime Trilogy, etc. the console is EOL and these games aren't becoming less rare. It's just how it goes. Look at PS1 games like Tales of Destiny or other first party bungles like Panzer Dragoon Saga.