Of course, Defense Grid is able to do this because it's split into separate, unrelated levels. With games such as Fallout 3, Borderlands or Oblivion, more expansive--and expensive--DLC makes the most sense. However, I always find it offputting to know $30 of DLC will soon be released for my brand new $40 game. I waited for Game of the Year editions before buying Oblivion and Fallout 3, and plan to do the same for Borderlands.
$10 is not a lot of money but it's past the point of a casual, impulse purchase for me.