The Litter Robot 3 is a well built and well designed workhorse. I think they cost a little more new than your budget, but they may have a model right around 500.

Depending on your comfort level with the idea and where you live, you can sometimes find them on the used market for a couple hundred dollars, or even occasionally a free non-working unit. Replacement parts are easy to get and install.

They have a WiFi model that can use an app to alert you when the tray is full or something unexpected happens, but it works just fine without the app or internet.