Not sure why you'd want to be tethered with an HDMI cable (video) and a USB cable (touch commands and power) and
possibly even a third DC power cable, but .......
If you've noticed, Apple doesn't make any touchscreen computers, only iPhones, iPods and iPads. There is a niche company that specializes in portable touchscreen monitors, GeChic, but to use the touchscreen functionality natively you have to use Windows 7/8/10 or you can download drivers for Linux and Android.
There are some people who have fudged around with a monitor like the GeChic and a third party program like Touch-Base UPDD, but results with a Mac have been patchy trying to get it to work properly and smoothly.
There may be something out there, but apparently Mac isn't the best candidate and "reasonably priced" probably isn't one of the attributes if you do find something. Good luck!
Steve