Necissity, need and the cost of value

Recently, I went shopping. I needed dress shoes for 2 weddings coming up in my family. So I decided I wanted something pretty and comfortable, as well as well built. Most of my shoes that I do own are really cheap. So I proceeded to be seduced, there really is not another word for these 4 inch leather heels by Ralph Lauren down from 90 to 60. blinded by beauty, craft and overall comfort, I purchased them with little thought.
It was only when I came home I realized i paid 70 with tax on a pair of shoes. 70, when I am desperately saving for a new( er) car before fall. ( long story but the car I'm driving now doesn't have a defroster/ working air vent)
After much panic, guilt, and almost crying; mostly at Amanda, I realized these are just a pair of shoes, and I can take them back, get a full refund and have no problem. Then, I found some spare cash that almost equaled the cost of the shoes. Now, the thought is to deposit this into the bank and use it to pay the bill for them, thus rationalizing the cost and value of the shoes.
Now, I find it terribly amusing that finding cash I didn't know I had ( I love when this happens, don't you?) can be justification to keep something I had prior decided as not worth the cost. What gives something value? Is it craftsmanship, want, desire, aesthetics, or is it something else all together?
What can we use to assign value? And where is it just impulse purchases or when is it an investment. Is Pleasure of object or ownership quantifiable?