I'm also looking for something like this. I see some on eBay for about $3.99, but I'm not sure I trust it. I know the Rocketfish Hard rear shell I had for the 1st gen mini doesn't fit the retina.
Lots of cases on Amazon if you look. I have been testing out the i-BLASON on my Air for the past few days and I am really liking the hard shell version with the anchor locks. It's more than three bucks, but how much was your Mini? It's worth the cost.