Adding Wifi+ Micro-controller to a device in an economical but production-able way
Unless you are talking astronomical volumes, it's just not going to happen in your price range with parts that are currently on the market. 

The cheapest wifi implementations tend to be no-name USB dongles (about at your total price point), but to use one of those you need to have a microcontroller with USB host capability, which all by itself exceeds your budget.

Something like the eletric imp device might approach what you want, but you'll see the pricing is nowhere near your goal.  It's possible that up at the top of the supply chain the actual parts cost of that module might be about your target, but to get that you would have to have their volume, and make your own run through all the design, certification, and manufacturing hurdles of making something like that from components, at the investment of a lot of cost and time.

(But this disappointing reality doesn't make your question a bad one to have asked)