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Last August I went to Lowe's and spent $693—money I was urged not to spend. The advisory came from my father. He never likes to be fussed over, especially if the fussing involves unnecessary expense. And his attitude hadn't changed even though he had just been diagnosed with leukemia.

But my father's health crisis left me feeling shattered—and helpless. So I bolstered myself by trying to control as much of the situation as I could. When I found medical studies showing that airborne mold spores can be fatal to a patient whose immune system has been compromised, I headed to Lowe's and loaded a cart with a powerful air conditioner (to replace an old, moldy one), two HEPA air filters (to catch ultrafine particles in the air), cleaning supplies, and more. Before the receipt could top $700, my boyfriend stepped in and put the second $220 HEPA filter on his credit card.

I knew that if my dad were there, he'd be grimacing. Watching the cash register tick up and up, I could feel myself grimacing, too, at the bite the bill was taking out of my modest, 30-year-old journalist's salary.

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But as anyone who has been there can tell you, the prospect of a loved one's death can drive you crazy. Maybe you should quit your job—if something terrible happened and you weren't there, you'd never forgive yourself. All you can see are the holes your loved one's absence would leave in your life.

My father's doctor had warned me about the slippery slope of this mind-set. He pointed out that scrambling to protect against every contingency could be harmful in its own right and that studies on air filters' effectiveness in particular were "inconclusive." Dad would be fine if he simply wore a mask outdoors, he said.

But this wasn't the doctor's father. He was mine. He was the one who had cared for me, educated me, and bailed me out of trouble a thousand times. He may have been careful with spending on himself, but he was quick to lend me money and time whenever I needed either. Now it was my turn to sacrifice for him, even if my efforts were a drop in the parental bucket.

When I arrived home with my Lowe's haul, Dad was skeptical, to say the least. Didn't the doctor say those HEPA filters were unnecessary?

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So I did what any daughter of a mensch would do: I told him how much I spent. Then I told him I wouldn't be returning a thing.

He gave me a hug. Thank you, he said, holding me tight.

You can't buy certainty, no matter how much you spend—or how hard you love. But you have to say yes when you get the chance, however improbable, to rent a little peace of mind. For me, that feeling of hope came cheap at $693.

Former Money writer and special projects editor Susie Poppick now covers personal finance for CNBC.com. Do you have a purchase you consider Money Well Spent? Email us about it and what it means to you at wellspent@moneymail.com.