I am not a betting man, but would happily put the price of a Surface Pro 3 that it is upgradable to 10, I would guess just about everything that runs full Windows 8/8.1 and most machines that run Windows 7 will be upgradable.
There are several (Maybe many more) already running 10 in the developers preview.
Add in that MS have stated that all Lumia phones will get the upgrade, and have given very strong hints that the Surface RT will also get it in some form or another, it would be a real shock that their flagship device would be unable to run it.
One thing we don't know yet is the price, my money is on WP's getting it free, I would also gamble that 8.1 users could get it free, and a charge being levied on W7 users.
I can only go on my original Surface Pro, stonking bit of kit and the Surface Pro 3 is far better, so it's just down to how much you want it.