While I wouldn't advocate spending £70 on a pair of chinos, they're likely to be a lot better quality than a £12 pair from Primark. There's a reason Primark is so cheap - nobody minds when their clothes fall apart after 3 washes. If a £70 pair of chinos had the same longevity, you'd be taking them back as faulty.
I'm very much a middle of the range kind of guy. Find the best quality you can for as little as possible, Usually this leads you to places like debenhams (Rocha John Rocha, Jasper Conran, jeff banks etc.) or sometimes Next. Very rarely Primark.
Interestingly I've found top man clothes are usually on a par quality-wise as primark. Yet they charge sometimes 10x as much.