We occasionally make the pilgrimage to Costco, and one of our favorite finds is the Weatherproof brand for socks, shirts and pants. For an early fall wardrobe upgrade we picked up the Weatherproof men's lightweight flannel shirt, an absolute steal at $9.99, on sale from $12.99
The flannel is a perfect weight for cooler temps at high altitude and hanging around the campfire in the evening. The print is good looking enough to do business casual with jeans or slacks. We've had good luck with Weatherproof flannel holding up to repeated washing and wear, so we're confident it's not cheaply made.
The only thing to be aware of is that they do shrink just a bit in the dryer, so either line dry or if you're normally on the edge size-wise, go bigger. Get them while you can.