What are a pair of fresh water 2010 Merc 850's worth? complete less headers,
under 30 hours.

To buy or sell? There still should be a good market for those motors. Do they have trannies, headers, etc. That's where the price goes higher.

Oh, those would go nice in my boat.

They are probably worth 40 to 50 I would guess without accessories, just guessing though. 850s are hard to come by. Would make anyone a great power package, easy to upgrade if needed.

40-45k for pair with no headers

You also have to take into consideration they are discontinuing them.