This statement is usually not true. The item in question may be pricey, but when one makes such a statement, more often than not, he or she has the required amount of money sitting in the bank. The actual issue is whether or not, one is willing to fork out the money to purchase the item.
Why is this bad then?
The phrase "cannot afford" is naturally self-demeaning. It erodes your confidence without you even realizing it. In making that statement, you are discounting your own merits and strengths before anyone even make any judgement on you!
We love discounts, but never discount yourself!
To put it in numbers, for example, if you rate yourself 10/10, and someone else gives you a 7/10, your overall "compounded" score will be 70/100 (derived from 10/10 x 7/10). However, if you decide to give yourself a discount before that, say, 8/10, and your friend rates you 7/10 again, you will barely achieve a pass with 56/100!*
So next time you see something out of your budget, do yourself a favor, say "I can afford it, but I choose not to."
*For those who have trouble understanding why the overall score is compounded, try looking it at this way: If you discount yourself, the amount confidence will be lowered and you will be presenting your "discounted" self to others. From the the other party's point of view, he will then judge you based on your "discounted" self. That is why the score is compounded instead of a simple addition.