I work in a jewellers myself, and here's the funny thing.... it all depends how stupid the girl is mixed with their class....
Chavy girl like Pandora, it's basically overpriced trash.
Tiffany is what a lot of working/lower middle class people just presume is a good standard. Still overpriced trash... a 9ct "silver" ring can cost around £170-£200 here.... seeing silver currently cost's roguhly 40-43p a gram and the average ring is between 1 and 6 grams.
Links of London is the only common sense(ish) group, good quality and expensive. Overpriced, but if you gonna do it, do it here.
Cartier, great quality, but so damn expensive! For those with more money than sense.
In all honesty try and stick away from the branded stuff, you can get 'identical' stuff for a fraction of the price. For example we buy one of our rings from a supplier who sells the exact same ring to John Lewis.... hardly a massive jewellery brand but servers as a perfect example. The ring is identical in every single way and is produced by the same person. We would have charged £2,500 while they charged between £8,000-£9000.
About 70-90% of the cost will be the brand, which usually means in reality all you'll get extra is a fancy bag.