how much is my MacBook air worth?

Browse through various only sources such as Ebay, Craigslist, etc. or simply Google and see what people are asking for similar devices.

Ultimately what determines what it is worth is where you live and how quickly older Macs sell where you live.

Hello ~Take a look here:

Trade in your laptop - Apple Official Site

~Katana-San~

Potential buyers determine its worth based on its age, configuration, condition, etc.

Be sure to properly configure it for sale: What to do before you sell, give away, or trade in your Mac.

Depending on your configuration, it might sell for about $450-$500.