Apple offers larger 256GB capacity for both 9.7" & 12.9" iPad Pros
For the first time ever, Apple's iPad lineup is now available in jumbo-sized storage capacities of 256 gigabytes, for both the 9.7-inch and 12.9-inch form factors.

The iPad Pro lineup is available with 256 gigabytes in the 9.7-inch size for $899. Adding cellular connectivity ups the price to $1,029.
And the 12.9-inch iPad Pro has also been given a new 256-gigabyte storage option starting at $1099. Adding cellular to that model brings in a top-tier price of $1,229.
With the new flagship 9.7-inch model being rebranded as iPad Pro, the iPad Air 2 still remains, but carries a lower $399 entry price for 16 gigabytes of storage. Users can get 64 gigabytes for $499, but the 128-gigabyte iPad Air 2 has been discontinued.

The iPad Pro lineup is available with 256 gigabytes in the 9.7-inch size for $899. Adding cellular connectivity ups the price to $1,029.
And the 12.9-inch iPad Pro has also been given a new 256-gigabyte storage option starting at $1099. Adding cellular to that model brings in a top-tier price of $1,229.
With the new flagship 9.7-inch model being rebranded as iPad Pro, the iPad Air 2 still remains, but carries a lower $399 entry price for 16 gigabytes of storage. Users can get 64 gigabytes for $499, but the 128-gigabyte iPad Air 2 has been discontinued.
Comments
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How much space does the OS take up compared to Windows? How much space for the typical app?
To start with the 16GB starting storage for iOS devices is too small, 32GB should be todays minimum.
Then the cost increment for more storage could be "just a bit" lower, considering SSD costs today !
MS Office for iOS is about 5 GB
Windows Office Pro requires 3 GB storage space. Probably less because I would imagine that many features of Windows Office are built right into the OS itself whereas for iPad the entire suite has to be completely self contained.
It really depends on users' intended use. For you, it may not but for others it may. My wife never touched her laptop again since I bought the first iPad. I wouldn't say I can give up my Macbook Pro for iPad Pro because iPad doesn't meet my PC intended use. However, I would give it a trial if the Smart Keyboard comes with Magic Pad integrated. Apple, hear that? Integrate the Magic Pad into Smart Keyboard and many people may drop their Mac.
http://appleinsider.com/articles/15/06/11/first-look-ios-9s-new-cursor-controlling-gesture-keyboard-on-ipad
"Replacing" depends what you do and how you do it; many of those ultra light laptops had big storage but it was dog slow and despite in theory having a fast CPU/GPU,the actual performance was still slower than the Ipad Pro. That's were Apple controlling the whole stack comes in.