I'd consider Topman and River Island to be on par. You'll pay £40 for a pair of denims in Topman.
I have never seen jeans for more than £25 in Topman, whereas Next and River Island are always around £35.
Topman is a sort of glorified Burton (normal plain cheap clothes, bit like Newlook but for men, or obviously Topshop), Next is a 20-40's "normal" clothes shop and River Island a bit of a Richard Hammond 20-40's clothes shop IMO.
EDIT: I take that back, I just looked on topmans site and they have quite a few £40 Jeans. :s I obviously manage to miss them every time I go into the shop!
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