The Verge reports, that the 64GB/128GB version of the Surface Pro out of the box offer 23GB/83GB of free for use space respectively, so 41/45GB are already in use by the OS/ built in aps / recovery partition. This has been confirmed by a MS spokesman.
Some of the space can be freed up by deleting the recovery partition, but MS recommends to create a bootable backup on a USB storage device in that case.
I know several people have shown interest in the Surface Pro on this site, will you rather go with the 128GB now?