It is REALLY hard to say what it's worth without seeing it, functioning it, and inducing it thoroughly. The reason, is because there are so many things they can be detrimental to the outdrive, that you wouldn't be able to guesstimate at or notice with the ***** eye without knowing thoroughly what you're looking at and for. This isn't exclusive to the 280, but the entire Aquamatic/AQ family. These are the only drives im extremely familiar with and work on nonstop. Hence — I've learned the hard way one too many times.
So, the reason you're seeing prices all over the map in the used market come down to a myriad of reasons - 1) familiarity. There are some people who aren't all that familiar with these drives, so they just mimick what they see on Ebay, 2) some think they're sitting on gold, and they're just waiting for the right sucker to come along and fill their pockets (you know how some people just continually throes money at a problem to fix it, no matter the cost), 3) others know the things I listed earlier above and they may realize the drive only has a few hundred hours left on it or it's on its last leg, while others may have fully rebuilt theirs to proper VP specs with thorough documentation and are getting top dollar. The list goes on.
So, long story short: it's worth whatever someone is willing to pay. But, you will typically make more money by parting one out as compared to selling the unit as a whole. Much more of the market is looking for used parts and pieces to fix their setups VS those looking to outright but an entire drive.
For example: I'm looking for a clamp ring and u-joint drive assembly/shafts from a 280 to replace mine. But, I don't need a whole drive so it makes much more economical sense for me to just buy parts and pieces (as I'm sure you more than likely already know this about markets like these).
Depending on condition overall after thorough inspection of known trouble areas, you could be looking at $1000 to $3500+, just based on what I've seen throughout the past few years. But, no telling really without a hands on inspection and etc... Hopefully this helps at least somewhat. :/
Edit: flywheel housing, the V8 red ones, seem to go for around $100-150ish without PDS and shafts, to $250-$300ish with PDS assembly buy not rebuilt, and around $400-$500+ish for refinished/repainted/new PDS assembly, seals, and bearings. This is JUST for the flywheel housing & pds setup mind you.