A Microsoft representative said, in a statement,
The 128GB version of Surface Pro has 83GB of free storage out of the box. The 64GB version of Surface Pro has 23GB of free storage out of the box.23GB for user files and programs isn't much when one looks at the size of some programs.
It's hard to figure out why the 128GB version only has 83GB of storage. When one does the math, you'd assume it would have 23GB + 64GB of 87GB of storage (and in this case, every GB counts). However, Softpedia argued with Microsoft, saying that the 64GB version would only have 19GB free, which jives with the 83GB total for the larger storage device.
At any rate, it seems that Windows 8 will suck up about 45GB of storage. Let's not forget all the temporary storage Windows uses to cache files for various reasons (browser, installations, etc., etc.) and you can see this could be a problem.
Microsoft also responded with a few suggestions:
Surface Pro has a USB 3.0 port for connectivity with almost limitless storage options, including external hard drives and USB flash drives. Surface also comes preloaded with SkyDrive, allowing you to store up to 7GB of content in the cloud for free.
The device also includes a microSDXC card slot that lets you store up to 64GB of additional content to your device. Customers can also free up additional storage space by creating a backup bootable USB and deleting the recovery partition.
We'd actually more concerned about program installation than data file storage. Thus, for us, the microSDXC card is the best one. Note, however, that it would need to be very fast if an end user wanted to run a program from it. Thus, we would recommend you get no less than a Class 10 card. We'd also recommend using highly rated and name brand cards.
The lack of available storage on the 64GB Surface Pro should come as no surprise to Windows users. The Windows 8 Pro used on the tablet is the same as on your familiar currently shipping Windows laptop, and no one would call that OS petite.
The need for a microSDXC card adds more salt into the wounds of those considering a Surface Pro tablet. It would seem the lowest-cost usable version is not a 64GB tablet + a Type Cover, or $1,029, but a 128GB version + a Type Cover + a 64GB microSXC card or $999 + $130 + approximately $65 for the card, or $1,184 -- and don't forget sales tax.