The Nifty Mini-Drive for the Mac Book Air (and the Nifty MiniDrive Pro for the Macbook Pro) is a new hardware project which raised almost $200,000 (at the time of writing) on Kickstarter, much exceeding its original goal of $11,000.
From its introductory videos and write up, it's an easy to use device that will allow a quick and simple add-on to the available memory in MacBook Pros and Airs, and as well as back-up solution.
All it does is to provide an intelligent 'adapter' that will take a MicroSd Card, which will then be inserted flush in the SD Card slot of the MacBooks, and used as an additional hard drive.
It appears that the price of the Nifty Mini-Drives will be in the vicinity of $30 each, and as the current price of a 64GB microSDXC SanDisk (as an example) is about $60, the total cost to increase one's memory by that amount is about $100 (inclusive of sales taxes or shipping, etc).
As a guidance price, the price of an 11 inch 64GB MacBook Air is $999 versus $1099 for an 11 inch 128GB MacBook Air. However, if one already owns a MacBook Air (or Pro) and wants a quick and simple increase in memory, this might be a smart alternative than to send it for an upgrade.