I originally referenced these in Drietz's thread about shorts for skinny kids (Shorts for Skinny Kids) as potential mtb shorts. After 10 months of use, I think they warrant a review of their own.
Anyway, I think the dark gray plaid version of the C9 by Champion golf shorts at Target might be the best MTB shorts available for under $50. The shorts are made of a tough polyester blend material, with a really hardy finish that resists staining and runs. The inseam is about 10 inches, and the legs are fairly narrow to avoid catching the seat.
Important to note, however, is that the single-color versions and the white plaid version are 100% polyester, while the gray ones are 90/10 polyester-Lycra, making the gray ones preferable for riding.
I have been wearing them with bibs underneath, and find them really good for mtb. The only improvement I could think of would be a flap for the rear pocket, which has neither a flap nor a through-button. In practice this hasn't proven a problem when I've forgotten my wallet in my pocket while riding, but still.
Best thing is that they're $28 and can be had for around $20 on sale.
C9 by ChampionĀ® Men's Golf Shorts - Plaid : Target Mobile