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Not sure what you consider not too expensive but I had an STM laptop backpack all through college and it was fantastic.

STM Small Aero Laptop Backpack

I also like a lot of Timbuk2 stuff but all of their laptop backpacks are designed for up to 15" macbook so your 13" air would fit but not necessarily be snug. I personally wouldn't worry about this as long as the compartment is padded.

Timbuk2 laptop backpacks

There are a ton of other bag companies I like but most of them have a pretty high cost.
 
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