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.
The current 30 day volumes for those sites at the time of this post...
(See
http://bitcoincharts.com/markets/ for current numbers).
6,136,000 USD for MTGOX
619,000 USD for TH
293,000 USD for BTCE
Now let's calculate their monthly income at 0.6% commission that is taken from both sides of the trade...
MTGOX... 6,136,000 USD * 0.006 * 2 = 73,632 USD
TH... 619,000 USD * 0.006 * 2 = 7,428 USD
BTCE... 293,000 USD * 0.006 * 2 = 3,516 USD
You are off by nearly an order of magnitude.
And that's a maximum; many big traders get discounts.
The lowest commission rate at mtgox is 0.25%, so let's use that as a minimum...
MTGOX... 6,136,000 USD * 0.0025 * 2 = 30,680 USD
TH... 619,000 USD * 0.0025 * 2 = 3,095 USD
BTCE... 293,000 USD * 0.0025 * 2 = 1,465 USD