There's a lot of paperwork to do this though and I'm pretty sure that most of those bitcoins from the seizure would be subjected to police auction, isn't that what they've done before with other bitcoins that they've seized from Silkroad? It's going to be cheaper when they do it in the auction though, last time that they've done it, the winner of the bid paid like half of it's worth in the market so I expect that's what's going to happen next time. The effects on prices will probably only be visible once the winner of that bid sells those bitcoins and introduces it to the circulation.