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.
Roast your own! There’s the startup cost of a roaster (there are some pretty impressive machines available now at around $500 USD) and from there you’ll be spending 1/2 to 1/4 for beans. It pays for itself quickly for any serious coffee drinker and you get a new fun hobby out of it!