A rough estimate of the money you'd need to take a position in a single stock would be:
(current share price * number of shares) + broker commission
In the case of your Walmart example, the current share price is 76.39, so assuming your commission is $7, and you'd like to buy, say, 3 shares, then it would cost approximately (76.39 * 3) + 7 = $236.17. Remember that the quoted price usually refers to 100-share lots, and your broker may charge you a higher commission or other fees to purchase an odd lot (less than 100 shares, usually).
I say that the equation above gives an approximate minimum because
- Commissions vary from broker to broker.
- If you submit a market order, you get the market price, which is whatever price the stock traded at when the broker filled your order. This may be different from the price you saw a few seconds earlier in Google Finance, Bloomberg, etc. This is especially relevant if you submit an order when the market is closed because the price you see when you submit your order is the closing price from the previous trading day. The opening price of the next trading day (which is when your order will be submitted) will likely be different from the previous closing price (this depends on several factors).
- If you submit a different kind of order, e.g. a limit buy order, your order may be filled at a different price than the price specified in your order for similar reasons to the above or reasons depending on the price of your order. In the case of a limit buy order, your order will be submitted at a price no higher, but potentially less, than the price specified in your order.
- If you start using more complicated orders, there can be more factors involved, e.g. margin requirements.
- Some brokerage accounts may require you to fund your account with a minimum amount, so even if it would only cost $100 to buy shares in a company and cover the commission, you couldn't open an account with only $100. That's a minor point, however, because many brokerage firms don't require an account minimum, at least for stocks.
- Commissions may vary depending on the stock. Some brokers will charge you higher commissions for buying pink sheet/over-the-counter stocks and/or stocks with a share price less than a certain small threshold.
However, I second the comments of others that if you're looking to invest a small amount in the stock market, a low cost mutual fund or ETF, specifically an index fund, is a safer and potentially cheaper option than purchasing individual stocks.