>>ape4+Uc
You're not going to find a decent one sub $500. Used devices can be had for less, but most of the older ones I've seen use terrible Java-based clients that are a major pain to get running on modern browsers. For multi-device KVMs you usually have to figure in the prices for the individual interface module dongles that plug into each computer, too.
StarTech makes a USB-to-KVM that looked promising to me, but it's still >$400: https://www.startech.com/Server-Management/KVM-Switches/Port...