Are Shih-Tuz with a brown or tan nose more expensive and why?

The color depends on the color of the dog. I quote the AKC standard: "Nose, lips, eye rims are black on all colors except liver on liver pigmented dogs and blue on blue pigmented dogs." Pink on the nose, lips or eye rims is a fault in all colors.
Unless you plan to show this dog the color should not matter. If someone is trying to sell the dog based on the color of its nose, I would be very suspicious of their motives beyond money. If you decide to purchase the dog please have it spayed or neutered.