Denim mania is surging across the fashion spectrum. At one end is the luxury brand Alaia with an Aegean blue, comfortable yet flattering £800 pair. At the other is JW Anderson’s collaboration with the high street brand Uniqlo and a £34.90 price tag. Both are proving wildly popular.

Alaia’s line has only just launched, so there are no sales figures yet, but demand for its Japanese denim is such that customers are advised to reserve certain styles in store or call ahead before visiting. At Uniqlo, the straight cut are said to be the most popular, on the front row of the most recent fashion weeks, and routinely sell out online. Blame the resurgence of 90s minimalism.

The question is, can you tell the difference? The Alaia jeans sit perfectly but Uniqlo’s slide on nicely first time too. The main difference is that after some wear, the latter have begun to lose shape in key areas.

Amy Leverton, who runs the denim consultancy firm Denim Dudes, says comparisons are tricky given the manufacturing methods differ. “Sagging and bumming is about a lot of things, including weave density and stretch content. But if a pair of jeans feels soft when you put them on, it sometimes mean the weave is [loose].”

It is in the next step of denim care that the gap between cheap and quality really shows, says Evoléna de Wilde d’Estmael, a co-founder of Faircado, a secondhand shopping app that focuses on sustainability.

“A £30 pair that bags out after a few wears pushes you into constant washing, which breaks down the fabric even faster and wastes a lot of water. It’s a vicious cycle,” she says. “Good denim holds its shape, rarely needs washing and actually gets better with wear. You end up with a completely different relationship with the garment.”

Still, for £35 you can probably live with a bit of sagging. And who in their right mind can afford a pair of jeans that cost the same as a flight to Mexico? High prices may come with a promise of quality, but that is not always the case, as this year’s digital product passports should help expose.

“Once every pair of jeans has to come with a transparent record of its materials, supply chain and environmental impact, it becomes much harder to hide behind a low price tag,” says d’Estmael. “A lot of fast fashion won’t survive that transparency, but for quality brands it’s a huge opportunity.”

Leverton agrees. “That’s why I only really wear secondhand Levi’s,” she says.

The rise of the luxury jean has been bubbling up for a few years. At Bottega Veneta in 2022, Matthieu Blazy created a pair of jeans made of leather that cost $6,900 (£5,100) – and sold out in two weeks.

Since then, costly versions have appeared on Valentino and Erdem catwalks. Margot Robbie’s Chanel jeans, pictured in a recent advertising campaign, are not available yet but are expected to sit somewhere alongside Versace (£2,310), Balenciaga (£1,550), Brunello Cucinelli (£1,200) and Jennifer Lawrence’s Khaite jeans (about £600) in terms of price.

Can a pair of jeans ever be worth £800? “Inflation has made it not completely abnormal for artisan denim made in Japan to cost more than you’d think,” Leverton says.

The sort of slow-made selvedge denim Alaia works with, which is traditionally made in Kojima, Japan, is wildly in demand. Yet production is struggling. According to the fashion journal WWD, and estimated 10 to 20 factories have closed due to “ageing ownership” and a lack of skilled workers.

“[That price] can make sense if they last, hold their shape and retain value on the resale market,” says d’Estmael. Factoring in raw materials, manufacturing and transportation, and the true cost of clothing reveals a startling economic illusion. “Obviously, the real sweet spot [is] buying those [same] pieces secondhand.”