For sure many cars and tracks will come as add on downloadbale content simply because just like any other company PD will want to milk this game for all it's worth. For example Killzone 2 just got 2 new downloadable map packs which sold £5 or something like that, those could have been on the disc but to start milking the game they released it as an add on.
I am sure Sony and PD are banking on those GT nuts whom will buy all these cars and what not as add on content, my guess something like £1 for 2 cars or something along those lines. Think about it though, if 100k people purchase that car pack PD have made another 100k which they would not have got were the cars on disc. LBP is probably a better example, every week a new skin releases for £2. :/
........basically expect a large amount of cars and tracks to come via downloadbale content.