Re: Will DLC cost extra?
Like said before, I'd probably pay 5$ for extra content, especially car bodies and fixtures. But maybe no more than it, especially if the pack isn't really big.
Otherwise, I actually could spend 15-20$ for Diesels and Wankel (20$ for both), and maybe 30-35$ for a major expansion like, you know, BIKES !

Just look at GTA IV and Saints Row :
GTA IV has released big DLCs for somewhat 20$. They sold very well as it was somewhat as long for the price as the main game.
Saints Row releases a lot of inexpensive stuff like 0.99$ a new and funny weapon. That allows us to occasionally pay for something, so we don't pay that much at a time.
Besides that, would there be a "Season Pass" for DLCs ? What about the preorder & collector editions ? Would they pay a little less ?