I suppose in the end, it's really up to Sony as to what, if anything, they'll charge. And no doubt they've done some studies to try and determine what the market will pay and how much they'll buy at each price point.
To be pragmatic about it, I think what people have to consider, is that the amount of work that goes into creating a new car, even a premium car, completely pales compared to the extraordinary amount of time and resources needed to create a new track. Tracks where significant effort has been extended to create not only a photo realistic environment but one that's technically accurate with bumps, ruts, ridges and curbs, all in their proper place, not to mention signage, walls, catch fences, pits, etc, takes many months of development. And when you consider the cost of sending a crew to obtain thousands of digital photos, engineers and artists working untold hours to get things 'just right', one really has to wonder what the price point is, where they can actually generate a profit from selling DLC.
At the same time, this poll shows us that many people, at least on this board, are simply not willing to pay a lot of money, or in some cases, any money at all, for new content. And Sony has to take this into consideration. That said, from a sales point of view, what would be considered a 'successful' or profitable DLC release? If they sell a single track for $5 and 250,000 people buy it, you're talking about $1.2 million in gross profit. Would that even cover the cost of creating a new track? (I really have no idea--I'm just speculating).
I guess the most likely scenario will be that Sony ends up packaging tracks and cars together. It helps off set the cost and it encourages people to make 'bulk' purchases.