Depends on the condition more so. I'd rather have a bike with a few more miles than one that was left abandoned in the garage. The price difference is negligable on kms, it's the accessories that add value. Our 658 had the same kms on it as our wrecked bike but came with the touring windshield, tank bag, OEM luggage and the maintenance CD, all for less than a no frills bike advertised now. We sold the excess gear for a grand which resulted in a real good deal on the bike. See tho that the valve check isn't immediately due cause that's $700-800 right there if you don't do it yourself. 20k kms for valves (20, 40 etc.) Fork oil change at 30k kms. Check the steering bearings also. Then again, get it certified at a BMW Dealer and you'll get all the info then.
My F800 sits in my country garage and is a fav with mice. Check under the seat for evidence and look down the snorkel if possible. They like to build nests on top of the air filter which the engine will eventually suck that whole mess into itself. I've repaired some wiring also from the little beggars chewing. Our 658 was fresh out of the 20k inspection so the filter was changed and bike inspected.
If you buy one, PM me and we'll see what stuff I can sell you.