Well, some quick math... Mt.Gox does volume of around 22490 BTC (current) multiplied by $100 equals $2,249,000 in daily trades. Their trade fee is 0.6% which is $13,494 but multiplied by 2 since they charge on both buy/sell side equals 26,988 daily revenue multiplied by 30 days is about $809,640 clear revenue per month.

They don't need to advertise and their hosting costs I'm sure are a small fraction of that. Holding up $2.9M of theirs would be an unfortunate blow, but I don't believe detrimental.