Estimating production costs is tricky from our outside perspective. It also varies from industry to industry, selling 2$-shoes for 200$ and than claiming that you cannot afford proper education, wages, safety-, environmental- and social-standards is one extreme...
Sennheiser is family-company and mostly engineer-driven in their professional department (don't start with their consumer crap made in China) - no need for growth at any cost, no debts, long-sighted thinking and investments.
The HD800 took 6 years to develop, they sold about 4000 pieces in 6 months and final assembly (not production of the components!) takes 45min - these are the known facts, so let's estimate given that they want ROI within 20000 pieces (over 2 years are in fact very long-sighted for the electronic industry):
R&D with 5 people (most likely more, but not always working on the same project) in 6 years @ 100000€/year + tools/equipment = 3.000.000€/20000 = 150€
Production Equipment One assembly line + testing equipment propably doesn't cost very much, let's say 1.000.000€/20000 = 50€
Materials not very much because most material is sourced as finished component from suppliers, let's say 5€
Tools which can only be used for the HD800, especially molding tools by the suppliers, let's say 12 tools costing 50000€ each in average / 20000
Supplied parts additional to the tool costs mentioned before, the material and their margin, let's say 100€ (the Neutrik plug alone costs over 3€ for a normal customer)
Assembly including every costs (maintaining workplace, power, wage, tax, margin) in Germany is usally between 50-150€/h (about 15-25€/h wage for the worker) for 45min = 75€
So the rough estimation would be 410€ - but it could be easily 100-200€ more or less without knowing additional facts.
That comes close to a fair price for a high-quality-product, I'm sure they're making more money with one of their chinese headsets - it's most likely not their big "cash-cow"...
By the way, Beyer and Sennheiser manufacture their high-quality drivers in Germany - not in China. In fact, Beyer even manufactures most mechanical parts themselves and Sennheiser sends drivers from Ireland/Germany to China to assemble their lower-end headphones.... The HD800 uses a South Korean cable (not the plugs) and Japanese microfibre for the cushions (which are made in Germany) - the rest is nearly entirly made in Germany.
Most workers from Sennheiser/Beyer are skilled crafstmen, they are educated/trained about 3 years (usually in corporation with a technical school and their own education department) before starting their work. Many machines are exclusive for Sennheiser/Beyer, some of them are even developed and made by them, they both have over 60 years experience - Chinese won't be able to manufacture headphones like the T1 or HD800 without re-thinking their mindset regarding quality, production, education and people.