Sharebuilder.com has a plan where you can buy fractional shares of stock on a monthly basis for just $5 a month.
Lets say you have a budget of $100 a month.
You can tell sharbuilder to take out $100 a month out of your account and buy $100 worth of XYZ stock a month.
Lets say that stock is trading at $40 per share. You could get 2.5 shares of that stock that month.
You can also split it up into 10 different stocks if you want for the same price.
Take 10 stocks you like and it will split your $100 equally among the 10 stocks buying you $10 worth of all 10 stocks once a month and the cost would not be per stock.You would just be charged that $5 flat fee for all 10 stocks.
Its the cheapest way to do what you are asking.