Friendlybrown, that 400 is a completely different animal than the aforementioned student model. The top hat and cane Bueschers are one of the most sought-after vintage horns, and play up to their reputation. I have a tenor version myself, so I'm a little biased.
With these types of horns, condition is everything. Depending on whether it's tenor or alto, condition of the lacquer, whether it's been re-lacquered or not, the pads, whether they are still snap-ins or not, dents, etc., the horn could span a broad range of value. For a tenor, for example, I'd guess it would range anywhere from maybe $1000 for a crapped-out relac with dents and shot pads, to perhaps $3000 for a very nice condition horn that's ready to play. FWIW, I paid $1900 for mine a few years ago in decent condition (70% original lacquer) and used but reasonable shape pads, ready to play. I probably overpaid a bit.
You can look to eBay completed auctions for comparables, there's a few out there. When you sell, you could sell on this board or put it up on eBay and take your chances.
Pete