If you can tolerate ankle clips, I'm finding M&S's current ultra-light chinos OK for cycling but I've only had them a few months. I went for the ultra-light because the ordinary ones seemed to have too chunky a seam bundle in the crotch, which is a common problem with trousers these days now that few people walk or cycle. I think they were £20 to £30.
In a similar price range were some Coolmax chinos from M&S's Blue Harbour range. Not as good - they're already going shiny under the bum and the pockets almost require you to dislocate your wrists to access them while riding.
Fifty notes might be tricky, although M&S did cycling chinos last year for a bargainiceous £12.00.
They started off much dearer. As well as a shower-resistant fabric, thicker pockets (anyone else find the riding action wears pockets out by rubbing over your thighs repeatedly?), slim fit and ineffective ankle-poppers to stop them going in the chain so easily, they had a pad over the seam junction, which is a mixed blessing akin to using a saddle with deeper padding in the centre. It's not enough pad to make it push on most of my saddles, though, so they're still in use.