Rebecca Armstrong: It’s the value - not the cost - that counts

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Over the course of two decades of avidly reading women's magazines, I have become versed in the shopping gospel according to glossies. For example, no matter your skin tone or hair colouring, there IS a red lipstick out there to suit you. You might need to buy 27 different tubes to find it but, like the truth, it's out there.

I've learnt that, despite the generous page acreage given to photo shoots of the latest must-have printed trousers/sleeveless jackets/lurex jumpers (although the only real must-haves in the world are food, water and something to keep out the cold and the wolves), the truly stylish reader ignores fleeting fashions and invests in the occasional high-quality trench coat or LBD. And depending on which beauty editor you believe, you can save a fortune/horribly dry out your skin using toothpaste as a spot-reducer. Because I've been a magazine junkie for so long, I forget that not everyone knows these tenets – something I realised the other day when my husband was weighing up whether to buy a particular winter coat. "Well, on a cost-per-wear basis, £120 on a coat is totally fine," I said helpfully, to a blank look.

He'd never heard of the cost-per-wear concept, whereby you can explain away the purchase of the most extravagantly priced garment provided you're going to wear it every day for the next decade (the CPW formula is the price of the item divided by the number of days you'll wear it).

Obviously, spending a month's salary on a handbag, meaning you can't afford proper meals, is bad maths, but working out the value, rather than simply the cost, of purchases can be quite useful. This became particularly obvious this week when I backed up my iPhone and saw all the apps that I'd blithely paid around a quid for appear on screen at once.

Some have proved to be brilliant value. I use the Tube Deluxe app all the time to get around London. Cost per use – negligible. And it's saved me fortunes thanks to all those cabs I didn't catch after not missing final trains. I'd pay twice over for Angry Birds, Whirly Word and Trivial Pursuit because they've not only entertained me for hours, they've kept my stepdaughter occupied on long car journeys. But not all apps are equally good value – while the Kindle app is free (bargain!), the e-books on it aren't, but as they're so simple to search and so dangerously easy and instant to download, at this rate my cost per use is averaging about £10. I might have to give up on the e-books and stick to the magazines.

i@independent.co.uk