The now defunct BitMarket.eu was one of the first Bitcoin exchanges. Compared to today's trading volumes, it was very small. I was not aware that this exchange was also the target of a successful hack (I wasn't a customer of BitMarket.eu). From what I remember the exchange didn't store Bitcoin itself, but merely connected seller with buyers - but maybe my memory is deceiving me. It would be interesting to know if the sum of $400K is the worth of Bitcoin stolen at today's valuation or if it is based on the valuation at the time of the hack.
Edit: After reading the article again it seems clear that the amount of $400K is based on Bitcoin's valuation at the time of the hack. So today's worth in USD is significantly higher.
Given the circumstances of the arrest, it seems clear that the guy was arrested for his other hacks and that the discovery of his involvement in the BitMarket.eu hack is merely a coincidence.
ya.ya.yo!
18,800 bitcoins stolen in 2012 and then 620 in Feb 2013. So, 19420 (they claim were hacked) * $668/btc (as of now) is almost $13,000,000. So you are correct, it was the valuation at the time of the hacks.