In reply to origamib:
One of the cheap options is Marmot Precip trousers:
http://www.blacks.co.uk/mens/104436-marmot-mens-precip-full-zip-trousers.ht...
They are usually in 35-50 pounds range. Assuming that they are similarly made as their Precip jackets, I wouldn't be the happiest to wear them, but they are quite cheap.
I use 3/4 zip trousers (Rab Xiom). They are great, but a little bit baggy and hence easier to puncture with crampons. Some wear on one knee as well due to kneeling down on rocks, I suppose. Still, they are great as they are probably the lightest 3/4 zip trousers.
