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There's tons of options out there. If all you need to fit is a laptop and some clothes... just get any bag really. Use some spray like another anon suggested.
There's not really any bacoaching packs that are made for laptops, because no one backpacks with a huge heavy laptop.
Cheap pack brands? Check out High Sierra, Teton Sports, and Kelty. All have many options under $100, none waterproof, although all with generally have a tuck away rain fly.
I have a high Sierra 65L pack. Great pack for the price, but it's huge, you don't need that big. For a laptop, and some clothes, 35L would be more than enough.
