One of the Best Discount Online stock broker is ING-DIRECT or Sharebuilder. You can buy stocks and trade for $4. They have No Account minimum and No inactivity fees. You can also Schedule investments on a weekly, biweekly or monthly basis.

Here is their trading costs…

Service Description Rate
Automatic Investments1
(Stocks, Mutual Funds, and ETFs)
Invest in Stocks, Mutual Funds and Exchange-Traded Funds (ETFs) on a recurring schedule or when you have the available funds. Subscribe to a ShareBuilder pricing program and reduce the cost of each investment you make in your Automatic Investment Plan.

Note: No-load, no-transaction fee funds purchased through the Automatic Investment Plan will not incur a transaction charge.2
Basic: $4 per investment
Standard: 6 per month included, $2 each additional

Advantage: 20 per month included, $1 each additional
Real-time Trades If you are looking to buy or sell a stock right away online, you can place a real-time trade. Market orders are typically executed within seconds of being placed (when the market is open). $9.95 per online trade3
Options Trading Accounts approved for options trading can buy and sell calls or puts or write (sell) covered calls on securities held in the account. $9.95 per online trade
+ $1.50 per contract
Mutual Funds Invest online in no-load mutual funds without paying a transaction fee. Our No Transaction Fee (NTF) program provides access to over 70 funds including the ING Mutual Funds. $0 – online only2
Invest online in no-load mutual funds with access to over 170 funds from leading no-load mutual fund families. $19.95 per online trade2
Phone Trades As an added convenience, ShareBuilder accepts phone orders for ShareBuilder-listed stocks, ETFs, and mutual funds. To place a trade with a Customer Service associate, call 1-866-590-7629 anytime between 8:00am to 9:00pm(ET), Monday through Friday. $19.95 Real-time and Mutual Funds trades
Other Services Express Funding Real-time trade commission + $6.95 per trade
Options – Early Exercise $30 per early exercise, no trade commission
Options – Automatic Exercise $20 per automatic exercise, no trade commission
Options – Assignment $20 per assignment, no trade commission

Here are thier Margin rates…

Margin Loan Balance Basic Standard Advantage
Less than $10,000 6.50% 6.25% 6.00%
$10,000 – $50,000 6.50% 6.25% 6.00%
$50,001 – $250,000 5.00% 4.75% 4.50%
$250,001 and over 3.95% 3.70% 3.45%