I found a seemingly-high-quality CO₂ monitor here for less than $50, or on Amazon or from the manufacturer's website for $70.

The most recent discussions of different CO₂ monitors on the market I could find on LessWrong^[1] were a few years old, and it seems that prices have come down somewhat since then. I found this positive review of a CO₂ monitor manufactured by Qingping,^[2] and then found it on AliExpress for about $40 (though it seems to have gone up by about $5 since I ordered it). The review found that its measurements agreed quite well with the more expensive Aranet4, and it uses a Sensirion SCD40 CO₂ detector, which is apparently well-respected.

I'm not entirely sure what the think of the evidence around CO₂ negatively impacting cognition, but it seems plausible enough to be worth spending <$50 on a monitor.