TradeKing is not the cheapest commission, if you really wanted cheap Zecco offers $0 trades. The problem you may run into with brokers like Trade King is that there are hidden fees you don't know about. For example if you buy too many shares at once or if you trade a stock that is too inexpensive in price. Personally I like Scottrade and TD Ameritrade as thy are flat-fee brokers. You may pay more but you get a hella lot more for your money. Some articles here on the site that may interest you,
TD Ameritrade Online Stock Broker Review
Understanding Flat-Fee Trades