The question is not whether it's absolutely affordable, but whether it's relatively good value compared to what's offered by the competition. Rockstar, for instance, is selling GTA: San Andreas for $4; 20% what this DLC costs. Mass Effect 3 gave away DLC of this scope, not once but three times. If you wanted to spend $20 on a multiplayer team shooter RPG, you'd probably get better value from Mass Effect 3 than you would The Dark Below.