Obviously it's good to have some sense of how much such things cost over time, but if you have to worry about small pleasures like a daily cup of coffee, you're in pretty dire straits... and I know that when I've so poor, I had no problem realizing how things add up, because it was smacking me in the face constantly...

This, incidentally is one reason I like cash (over debit cards or whatever). I like to palpably feel my money going away if I spend it, and have a physical sense of how much I have available... it really helps keep me aware of the flow of money.