Easy Answer: It depends on the stock and who you use to buy the stock.
The absolute lowest price you could buy a stock for (that I've found to date) is $9.96 – that would happen if you bought one stock valued at one penny through ShareBuilder (the trading arm of ING Bank) with the price of the trade costing $9.95.
So for less than ten dollars you could own a stock.
Don't get me wrong, I'm not recommending you buy a stock that's worth a penny, because that's not going to help you retire.
But to be able to own one share of Johnson & Johnson or Coca Cola for less than a hundred dollars…maybe it's not a bad place to start.
Back in the day, you had to buy stocks in packs of 100 shares. Easy to trade, easy to add up. But with the use of computers now, companies like ING allow everyday people to make a trade of 1, 5, or however many shares they want at the flat rate trade price of $9.95.
What you have to do to get the $9.95 price
It's pretty simple. Upload some money into your account two days before you're going to start trading. ING holds that money in a mutual fund (so you're in stocks already, just not individual ones you have chosen) – then when you are ready, you put in a "buy order" (it's a lot like buying something after you win an auction on eBay) and you will find yourself the proud owner of stock.
Make sure to check out the blog post on LImit Orders vs. Market Orders before you buy!
Jennifer Gniadecki is a freelance writer making finances easier to understand. She blogs at Beyond Mom. My opinions on stocks are just opinions, don't go all crazy in the stock market and sue me, k? Thanks! Oh…and never, ever buy stocks on credit.

Actually, with ShareBuilder's AIP trades ($4), the minimum purchase would be $5 ($1 worth of any stock + $4 trade commission). You will likely only own a partial share, but regular investments will eventually add up to a full share (if not more).
Hi Scott,
Actually, I was referring to a one time stock purchase. I wasn't recommending this as the best course of action, just trying to show people that may not be ready for Automatic Investment Plan (what you mean when you say AIP.)
And there is an extra charge for that money to come out of your account on a one time trade…so if someone isn't ready to let a company tag their checking account on a regular basis, this is the cheapest.
Mabye you're a more advanced trader than I am, but I'm all about making small purchases and learning as I go without making committments like automatic investing.
Not yet anyway!
Thanks for the comment.