Ok, so it ended up a bit over $80, but the basic components are:
I got lucky on the CO2 cylinder, and it was even aluminum, but they shouldn't run more than $50 at just about any welding supply store.
I made the diffusion reactor with an old Tap Water Purifier I had lying around. I epoxied an inlet, tangential to the cylinder, near the top. The water swirls around downwards, and then travels back up through a UGF tube back up to an exit at the top.
My bubble counter is a glass oregano spice bottle. I drilled two holes in the top, epoxied some lines in place and filled it with water. Had to seal the lid on with silicone, and it doesn't leak.
So far the plants are responding with intense pearling by late evening. It's been running for just two days so we'll see how much growth we see versus the old yeast CO2 method I was using. Just being able to remove all the clutter from the tank (reactor, tubes, powerhead, etc) is a nice bonus by itself!