I believe that is an 80's era design by Wegner. He designed quite a few chairs during this decade, which are not that well known. I think this chair might be in the Oda Wegner book, but since I am out of town, I won't be able to check until next week.
Wegner chairs of this era are hard to put a value to. Since they were designed/made well after the Danish Modern heyday, there were not many made. My personal opinion also is that these later designs are not as nice as Wegner's earlier work.
Having said that, I think 400 Euros sounds reasonable. If it was an early-mid JH/Wegner chair, I would jump all over it for that price if the condition was decent.
We don't usually discuss value around here, but as cdsilva said it is hard to put a value to something like this. I will go further and say it is impossible. It is a bit like the irony of the word priceless. It paradoxically means worthless and infinitely valuable. I can say that you can expect to never, ever, ever bump into another pair of these chairs. Therefore they are priceless. This is why you want them and why you don't want them. Should you elect to purchase them you will be able to look at it both ways. Either you paid millions less than you might have had to pay, or 750 euros too many. There will also be another way to look at it, which is that you paid exactly the value of the chairs at exactly the moment of the transaction, because that is the definition of value. (It is a very strange word).
You can check that they are authentic by finding a catalog showing the design. Or an old advertisement. This could be nearly impossible to find though. You might need to contact a Wegner's daughter.
But wouldn't it be awesome if it turned out that they were perfectly Wegner-esque chairs made by some random person who bizarrely put late era Johannes Hansen tags on them? It would be like a Han Van Meegeren painting. This would make them truly priceless.
It may sound simplistic, but not many were made because not many were bought.
All Danish wood furniture sales were down during the 80's, and my guess is that those who wanted a Wegner chair during that time went with the better-known better-designed earlier models. You don't go to a Stones concert to hear their recent stuff.
Danish Modern style was going out of style around that time, too---which is hard for a lot of us here to imagine ever happening, but it did. It wasn't the only factor but it was definitely one of them. Various styles of anything cycle in and out of favor with the general public---even classics that will always have a fanbase.
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