How much $$$$ do Bitcoin exchanges make? READ HERE:
Currently the top Bitcoin exchange is BTC-E which has traded $569,046,852.38 USD over the past 30 days. At a 0.2% fee on each side, that's $2,276,187 per 30 days on Bitcoin/dollars alone! They also make money from litecoin-to-bitcoin transactions and dollars to litecoin. Bitcoins to euros, ect.
check out other exchanges here: http://bitcoincharts.com/markets/