It's going to depend on what's out there for you to pick up at under $500. MacBooks retain their value and finding one for under $500 is certainly going to mean you're looking at an older device 2012 -2016. Take a look at what you can buy used from OWC to see some expectations.
You can consider refurbished 2016 MacBook Pro 13 inch, 1.1GHz dual-core Intel Core i5, 8GB RAM, 128GB flash storage for $500. It comes with an awesome Retina display and awesome battery life. It is as good as new as Apple refurbish and test the product as good as new ones.