If the DeLorean was your only incentive to buying the game, you're probably better off not getting it, as the novelty of the car wears thin pretty quickly. The game has a huge amount of content, especially for $20. If you REALLY wanted the DeLorean, the extra $4 is probably less than you spend on lunch every day.
You can find the original Burnout Paradise for $20. All you're getting in the Ultimate Box is the weak "pass the controller" multiplayer mode.