Denim is every man's wardrobe necessity. A good pair of jeans will get you through anything. Besides, they always got your back when you have exactly 60 seconds to decide what you're going to wear in the morning.
If you're on a thrifty mission, your best bet is Wrangler Jeans. Another strong contender for your wardrobe is Cinch Jeans. You might also want to consider Levi's Slim Jeans. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.