Add an inch, add a yard
It's amazing how much one's perspective depends on context. You'd never pay $3 for a bottle of water, but when you're at a football game on a hot day, all of a sudden the price is right. This weekend, I was at a friend's bachelor party. I spent some $150 over the course of the weekend--an enormous amount for a fellow who hates to even pay for his own lunch, but once I committed to spending $100 or so, the remaining $50 came pretty easily.
I suppose this is the same principle behind selling "accessories" with new cars.