I would agree to pay $40-$50 for the "engine" (and all subsequent patches, naturally) and $10 or so per car/track (again, that includes all future updates, if there are any). For about $100 I would have months worth of gameplay (Formula Mazda + Laguna Seca + Watkins Glen + 2 random cars/tracks). But they would have to be mine forever, none of that subscription business.
EDIT: actually, if I found out the cars and tracks were THAT good, I could probably pay $20-25 for them. But I would like to own content.