Ranchsliders are good to ride in, plenty of room, but they are very very baggy! They are made of a pretty tough material (heavy duty cotton), so are a little hot in summer. I wouldn't recommend them for mtb use - too heavy, but are ok for commutes.
Kona make more mtb-specific pants - I've never tried them but they look quite good. Also worth looking at are very lightweight pertex windstoppers over normal lycras. They are suprisingly comfy and practical. I think Endura make some.
The cheapest combats are, unsuprisingly, from your local army supply shop. Very cheap so good for mucking around in, but they often have a little drawstring at the bottom that might be nasty if it wrapped in your chain.
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