in reality the cost of these windows is much less as the materials used are incredibly cheap to produce compared to the extensive sourcing involved in wooden and timber frames.
See more at https://www.compareglazingprices.co.uk/replacement-windows-cost/
When those who buy to renovate and sell it on get it wrong, they can lose a lot of money. Those with no or little experience and those who mismanage budgets tend to be the most at risk of failure. However, there are some great tips for those who want to give it a go themselves.