In reply to duckweed:
Two factors really, Simond is owned by Decathlon and they are distributed exclusively through their stores which means no extra distributors costs and shop costs, so they can retail at a much lower price. They also have a very limited R&D spend and tend to run lines for a long time. Then the materials are as Hema says a little less spendy and features not quite as good. But overall my experience of Simond has been that it is good functional kit at a fraction of the cost.
Osprey, well I used to agree with Hema, until I was looking for a new pack hand ended up buying an Exposure 35 pack. It quickly became my favourite pack and has remained so for the 10 years i've had it. I have bought others since, for example a Montane, but the Osprey is just well designed, comfortable and works in all conditions. The Montane, all the buckles broke, the fabric wasn't hard wearing, the ice axe holder which looked like it would be excellent IMO was total tat... really wasn't impressed at all. Hopefully they have improved since I bought mine... But back to the point, the mutant is their most stripped down pack - if it had existed when I bought mine, that's the pack I'd have gone for and TBH I wouldn't hesitate to get it if it was at the right price... £40 for a pack is not much really...