No one really knows, so it's all up for sepculation. I hope they add in DLC, there's always new cars coming out, and I'm sure it's easier to make a new car for DLC than adding it on a blu-ray disc with other content.
So my money is on that we will be getting DLC for quite awhile.
Edit- Pretty sure they will price DLC, major reason behind that is because they KNOW that many people will buy it. You'd be amazed how much time and money consumers will invest JUST on the stuff got GT5. I'm just waiting to buy a G27 and will be building my own fold-away cockpit very soon this summer.