I would not pay for add-ons, I mean you already bought the game (if you are licensed), I mean it's just something extra to make the game better, so make the game more worth the money
I do not think paying for add-ons is a good idea. I feel certain ones will give people that pay an advantage in racing online and would not be fair to the people that paid for the game but do not buy the addon for one reason or another. Other add-ons are not significant enough to be sold individually...just my opinion