Big-name retailers have defended selling clothes for £5 or less, saying their ability to sell clothes so cheaply is down to business models.
MPs investigating the impact of so-called "fast fashion" asked the firms how they could justify such low prices.
Primark's spokesman Paul Lister said the firm spent nothing on advertising and had tight profit margins.
Representatives from brands including Boohoo, Misguided, Asos, Burberry and Marks & Spencer also gave evidence.
The Commons environmental audit committee (EAC) is examining the impact of clothes production, especially those items produced cheaply and quickly in response to trends - known as "fast fashion".
Labour MP Mary Creagh, chair of the committee, asked Primark's head of ethical trade and environmental sustainability, Paul Lister: "How can you justify selling T-shirts in your stores for as little as £2 or £3, and how can you be making a profit on those?"
He replied: "Primark has never done any significant advertising at all, and that can save us in any year £100m to £150m, compared to some of our larger rivals. That goes straight into price. That keeps our pricing low."
"It's our business model that takes us to a £2 T-shirt."
On waste, Mr Lister said Primark had very little unused stock and was planning to launch a take-back scheme for consumers next year, where old clothes can be returned and used again by overseas charities.
Ms Creagh suggested that by making garments so cheaply, they were being devalued.
But Mr Lister insisted: "Every item that we make, we're looking at durability… we are proud of the quality and durability of our garments, they're not built to throw away."
What is fast fashion?
The term describes our high rate of fashion consumption fuelled by the availability of new and cheap clothing.
Producing clothes requires climate-changing emissions. Global textile production produces 1.2bn tonnes of carbon emissions a year - more than international flights and maritime shipping.
Last month, MPs on the EAC concluded that the fast fashion industry was a major source of the greenhouse gases that are overheating the planet.
MPs believe that the throwaway nature of fashion is also fuelling fast turnarounds among suppliers, which may result in poor working conditions.
Elsewhere, Carol Kane, joint CEO of online fashion house Boohoo, was asked how the company could sell dresses for as little as £5 when the minimum wage was £7.83.
She said this only applied to a small number of dresses intentionally sold at a loss, to drive more traffic to the site.
Ms Kane, asked if consumers were now too accustomed to cheap, disposable clothes, said: "I believe this all comes back to consumer demand. I've been in the industry for 32 years, and in that time I've seen prices decline."
Speaking on the same issue, Jamie Beck, from the Arcadia group, which includes Topshop, Dorothy Perkins and Burton, said: "These garments aren't designed to be a disposable item, to be bought for [just] a holiday. They're designed to be long-lasting."
During the hearing, high-end fashion brand Burberry also defended criticism from MPs for dumping clothes.
Earlier this year, the firm was strongly criticised for burning £30m ($40m) of stock. It admitted destroying the unsold clothes, accessories and perfume instead of selling them off cheaply, in order to protect the brand's exclusivity and value.
Leanne Wood, Burberry's chief of corporate affairs, told MPs the firm was "committed" to stopping the activity, but added: "It is an industry practice. We're the only luxury business that's reported it in their accounts… but it is something that happens in the industry."
Boohoo, Misguided and Asos were also quizzed on relationships with suppliers accused of exploiting workers in Britain.
Paul Smith, head of product quality and supply at Misguided, said the firm had cut the number of businesses it worked with in Leicester - where many of the factories are based - from 35 to just 20 due to concerns about pay and conditions at some sites there.
After the hearing, Ms Creagh said: "Evidence we heard today justifies our concerns that the current system allows fashion retailers to mark their own homework when it comes to workers' rights, fair pay and sustainability.
"Marks and Spencer are supposed to be a leading light in corporate responsibility, but even they pulled out of a scheme seeking to achieve living wages for garment workers through collective bargaining.
"Boohoo did not convince us that it had a grip on the potential illegal underpayment of their Leicester-based workers."
She added that it was "shocking" to learn during the hearing that Misguided staff who went to check on conditions at a factory were allegedly assaulted by its owners, adding that it "begs the question - what on Earth was going on inside?".