I believe it's probable that they'll overpay a little bit.  It'd be prudent for both parties; you can't just buy 30k BTC @ X price in one action, the price will probably go up way before you hit 30k, and the FBI gets to charge a premium for their efforts.  

Plus the fact that it's an auction, we could see some seriously crazy shit happen.  Overpaying like crazy to drive the price up for a massive bubble, and it's likely whoever wins has massive amounts of Bitcoins already, so the chance they'll dump all-out to fiat is low; anyone heavily vested in Bitcoin wins from that auction.

On the flipside, let's say the FBI under-prices them at $300-$400?  They could negatively influence Bitcoin's price & viability, at the expense of cash (and as I write that last part it seems ridiculous for them to give up millions to try and dismantle something they should know they can't dismantle with just a few million dollars).