Right now I see on mtgox the lowest price as 105$ and the last price 165$ but when I want to buy by the price of 105$ I can not do so, the only option is 165$.
Is there something I understand wrongly?
That was the lowest price in the past day, not the market price.
Look at this chart: https://data.mtgox.com/api/0/png/24hours.png?Currency=USD The bars represent the current buying price, and the current selling price.
Right now there's a big difference between the price that you can sell bitcoins at, and the price that you can buy bitcoins at. This is called the spread, and it occurs because people aren't sure how to value bitcoins right now.
You can only buy Bitcoins at a price if someone is willing to sell them at that price. That a particular price is reported means something, but not necessarily that anyone is willing to sell or buy at that price.
In a healthy market, the buy and sell prices should be almost the same and there should be plenty of people willing to sell or buy at very close to the price at which the last transaction occurred. However, in this particular case, technical problems at Mt Gox caused the market not to be healthy and this general rule did not apply.
In any event, the "low" price just reflects the lowest price at which a trade occurred at a particular exchange in a particular time interval.