Hi there! I was in your same position a few days ago, when I ordered my YI 4K.
I noticed your exact same thing. What I also noticed is that the 10 bucks waterproof cases are different from the original, $40 one. Since waterproofing is a very delicate thing I did not want something that potentially would not protect my camera well. Going with a cheap case there's a chance you lose much more.
What I did, to save some money, is order a bundle from an AliExpress seller. I first contacted the seller and asked if the waterproof case they were bundling with the camera was the original one and then placed the order.
Result: I paid $25 for the original waterproof case (you can see the picture in
my thread), which, as you saw yourself, costs about $40 with a slow (at least for me in Italy) free shipping. I got my package (camera+case) in a few days, DHL in incredibly fast.
My suggestion is to not take the cheap route and choose the original waterproof case - for me personally there was too much at stake to save about $10.
