There are some devices that do this, but as you’ve already noted, they’re crazy expensive. There’s cheap(er) ways to accomplish the same thing, though.

Assuming you don’t need to do any heavy-duty computing on either screen, you could probably get away with a very cheap laptop for the primary display, and something like a Wacom display tablet for the touchscreen element. I’m not sure how compatible such touchscreen displays are with Chromebooks, but most Chromebooks easily have Linux installed on them, which I’d imagine has much better support. Or pretty much any low-spec Windows laptop would also work just fine.

You could probably get away with a decent setup like this for the $500-600 range. The tablet could rest over the keyboard of the laptop, or if you’re clever with drafting and have access to a 3D printer, you could probably easily print yourself a mount of some sort to secure the touch display over the laptop keyboard to avoid pressing keys and give it a more intentional design. I don’t know how well using an on-screen keyboard would feel, so something modular like this, where you could easily expose the real keyboard without too much fuss, may be a good route to take.