I found the northface merced cargo water shorts on sale at woodbury commons(in ny) northface outlet for $20. Not sure if other northface outlets have them but worth a look.

Here is a page of what the shorts look like:

I know they aren't specifically geared for mtbing, but as a over short paired with nice padded shorts they're great. The material is light, quick drying stretchy and pretty durable, so for $20 buck you can't go wrong.