Now look at the volumes in dollars. The standard 0.6% commission rate yields about $8000 per month for Mt. Gox, $810 per month for Tradehill, and $384 a month for Btc-E. And that's a maximum; many big traders get discounts.
Average volume for the past month looks like about $150k USD/day, which at 0.5% commission, is $750/day, or $22500 per month. Are you sure you were looking at volume in dollars, and not volume in bitcoins? $8000 per month / .006 = $1.3M per month, and they had $600k in currency volume yesterday alone.
$22500 per month is enough to pay $5k for rent (it's a shared office), $5k for hosting fees, and a living wage for 3 people (I'm assuming they can pay employees equity for the rest). They could have easily paid cash all their hardware back when they were making $5,000 - $10,000 per day back in May/June/July.
Sure looks to me like Bitcoin can support at least one exchange, even though it's no longer the heady days of June. And they could have easily taken $250k in investment money in June had they wanted to; I'm sure their phone rang with offers, and they had the choice whether to accept it or not.