It would seem logical that at the last auction, the cost of securities is the price at which it currently will sell, but it rarely happens.
Securities market (or its trading price) is based on bid and ask prices, not the price of the last transaction. Investors can use the last auction price to gauge where the market is and what people have done recently, but once this price is published, it is not the actual price You pay, if you decide to buy security.
When You place a market order, you ask for the price of the market, which means you have to buy at the lowest ask price or sell at the highest rates that are available for the stock. You can ask your broker for these prices – they are usually given to you when you request a quote.
Also, if you really want to buy or sell a stock at a certain price, it may be more appropriate to use limit to do so. Thus, You can be sure that all your buy orders will be executed at a price which is equal to or lower than a predetermined price level. Conversely, a sell limit order will ensure your order to sell is executed at a price that is equal to or higher than the price level that you want.
For more information, see “basics of the BID-ask Spread.”