SA Andersen Chair  

Condition + rarity + desirability (current market) = monetary value.
Any repair will diminish value with degree depending on quality of the repair and other factors listed above. There's really no hard, fast rule by which to calculate this. A photo of the repair might help.
Gorgeous chair, though. Congratulations!

tktoo,
Many years ago a friend and dealer suggested that I try not to purchase "projects".  His opinion was to spend more for better condition and be satidfied.
In the "modern design world" I have not yet come across many "rare" items.  I have seen and own what I consider "difficult to find" pieces.   My rule is "rare" is one of one and "difficult to find" is more than one.
I have twice required to help of an AIC or IIC qualified company to correct a problem.  I met Thomas Venturella by chance while visiting the Morse Museum.  He later restored a small area on a Mendini mirror by Edizione Arte.  When Caper, my dog, chewed the beak of a decoy I contacted the Havre De Grace Decoy Museum and their "contracted" restoration person corrected the problem.
I agree that repairs are not advisable and restoration is a last option solution.  Do not purchase "pedestrian" projects.  
tktoo, your posts suggest to me you may have been, are associated with, or have access to conservation and restoration knowledge and abillty.  This is good.  I enjoy your posts.

Thank you, niceguy. It's always nice to receive a compliment. Mostly, I just try to be helpful here when I can.
Not to argue, but unique is unique. I consider "rare" as limited edition with few, if any, examples in circulation.
My background is in museum exhibitions and I've been lucky to have worked closely with many talented conservators, curators, registrars, and collections managers over more years than I care to recall. I've handled everything from Rembrandts and van Goghs to to George Washington's hair and mummified human remains. Neophytes like to toss out insurance values here and there, but their silly numbers don't impress me.
I'm now a freelance "preparator" (for lack of a better description) and continue to provide a wide range of services to museums, collectors, and the occasional dealer. Been making a lot of object exhibit mounts lately, but I'm not above hanging framed works or building pedestals. It's hard to admit that I've gotten perhaps a bit old for painting galleries and hunking art crates, though I'll still do that, too, if a client is desperate enough to meet my rates!

Thank you very much for your thoughts.
Still not sure if we're good with that chair and the price we paied for it (EUR 800) or if we better return it due to the broken/repaired seat part. On the internt the price for this chair seems to be at least EUR 2'000 or more and this for a chair with replaced/not original upholstery. 
Attached a are a few pictures of the repair.
How about replacing the entire wood piece that got repaired? It's the one that has the logo on it. Will this make it even worse?
Also the rubber straps are a bit "dry" and will need to be replaced soon. Well I guess this is normal since it's a rubber part and obiously rubber won't last for ever.
Thanks in advance for your help.
Reto
 
 
 
 

Okay, that counts as a hack job. Any half-decent restorer could do better whilst asleep.
That said, if the damage/repair wasn't disclosed prior to sale, you may have grounds for complaint/discount/return/refund. Satisfaction depends on the circumstances and/or terms of the transaction, of course.
If, as you say, you paid less than half the going retail for the chair, and if you are happy with it otherwise, you might consider having the repair redone by a competent restorer. It doesn't look like it would be all too challenging or expensive. If it were mine, I'd probably get a price estimate for a proper repair job and use that as a bargaining chip when negotiating with the seller.
Oh, and I would want to repair and keep in place the part with the branded mark.

tktoo,
I am always willing to "pay to play".  Do you have a site that would include contact information.
I have no "need" at this time, but I do believe it is good to have the information when it is required not a "mad hunt" later.
 
 
 

Thank you tktoo & niceguy!
Any more coments on value on this chair would still help. 
Thanks!
 

No idea about the value, but I would be concerned to sit in it given the absolutely wretched repair job.  And a chair I am afraid to sit in for fear of breaking it is not one I value highly.
It would be important to have a well done repair because the break was along the groove that holds the rubber straps, so it needs to bear the weight of the person sitting in the chair.

Since you seem to be looking for a simple answer, I'll give you one: you paid too much for that chair with that damage.
If you didn't ask thrice about its value, then I would have assumed that you bought the chair with that damage because you wanted to enjoy it, and not because it was a deal.