I've had good luck charity shopping for Longchamp. I bought a large metallic pliage for £30 and I found this bag in Switzerland for a steal.
It was no pricier than the other bags on the rack. It looks like authentic Longchamp, smelt like leather and the quality was to be expected from the label. Not familiar with this model, I relied on the hallmarks of an authentic Longchamp-the quality of the stitching, positioning of the pattern of the lining (pictures centered), zipper pulls and stud branding and serial number tag. It even had a care booklet inside the pocket. Real or not, CHF 4 or USD 4.23 was a bargain.
Nowadays anything high end usually gets their own designer section with pricing to match the labels. I'm so pleased have snagged a lucky find in Caritas.
I identified the bag from the Longchamp website. When I tried it on in the Regent Street store, the bag was bigger. I was told last year's model was smaller, exactly the dimensions of my bag. Longchamp doesn't stock it anymore but Selfridges still sell this for £175!