"I can't afford it" has become the polite euphemism to decline a salesperson's proposal, regardless of the real reason why the customer might be disinclined to buy. It has also become an attitude - a state of mind that revels in the perception of scarcity. Even if something (say, health insurance, or proper maintenance of a car) is clearly a sound investment that will likely save big money on the back end, someone who has a resistance to such a purchase will say "I can't afford it." Or worse yet (and I've seen this tragedy unfold more than once), someone who is uncomfortable with their perception of costs will mentally shut out the benefits. Case in point: an outgunned family member (often Dad) has been persuaded, through much cajoling and hours of puppy-eyed pleading, to splurge on a trip to Disney World. Now they're there, and Dad is so busy grousing about every penny they spend on a meal or a souvenir that he completely misses a wonderful vacation with his family. This scenario happens at all economic levels. It's not about money. It's about an attitude of scarcity that somehow followed Dad to Disney World when it really needed to be stuffed into the bottom of his dresser drawer back home.
My point is not that you should get crazy and ignore your budget, nor that you should ever spend money when you are uncomfortable about doing so. But when you say "I can't afford it," ask yourself what you really mean by that. There are actually five different reasons why people decide to buy or not to buy: how they feel about the salesperson, how they feel about the company he or she represents, how they feel about the product or service, how they feel about the price, and how they feel about the timing. By asking yourself which of these things doesn't fit you, you will likely make better buying decisions at the same time you set yourself free from a cash crunch that may well be as much about your attitude as your bank balance.
And as for the trip to Disney World (or any other treat or luxury you elect to invite into your life), assess your bank balance first, then your desire -- and if you decide you can afford it (because it's worth not just your money, but your time, space, and attitude investments as well), make the full investment. Only when you give your full attention, and your full intention, to the experience will you truly reap the benefits you are buying.