From an earlier thread I did some math:
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.
I calculated about $809,640 clear revenue per month for Mt.Gox. I'm sure they have decent expenses but I would think they're a fraction of that. If Mt.Gox was a product manufacturer then I could see a problem, because holding up capital delays payments to suppliers etc. That could impact business. Actually Mt.Gox only deals with moving money around. Tying up 5M of theirs would be a big blow to profits I believe, but would not make them insolvent, nor inhibit their ability to continue operating.
Also, they plan to introduce Litecoin trading soon which I think will significantly boost their trade volume and of course fees and revenue.