A cheap CO2 meter can be perfectly useful, but not every meter that displays CO2 levels actually measures CO2. The difference lies in the type of sensor, and that largely determines how reliable the figure on the screen is.
NDIR sensors are the reliable foundation
Most serious CO2 meters use an NDIR sensor (non-dispersive infrared). This measures CO2 directly by detecting how much infrared light is absorbed by CO2 molecules in the air. This is the technology also used in professional measuring equipment, and a budget meter with a good NDIR sensor can be quite accurate, often within a margin of 50 to 100 ppm.
Note: not every "CO2 meter" actually measures CO2
Some cheap devices display an "eCO2" or "equivalent CO2" value. This is not measured directly, but estimated based on VOCs (volatile organic compounds) in the air. That can be close, but is substantially less accurate and reacts to very different things than breathing alone, such as cleaning products, perfume, or new furniture. Check the product specifications to see if the manufacturer mentions "NDIR", or only "eCO2" or "TVOC".
A simple sanity check: the value outside
A simple way to test a meter: take it outside for a moment, preferably not right next to a busy road. Outdoor air worldwide contains around 420 ppm CO2. If your meter shows a value between approximately 400 and 450 ppm there, it is reasonably well calibrated. If it consistently deviates significantly, that is a sign that the sensor is less reliable or needs recalibration.
Calibration: automatic or manual
Many meters have automatic calibration (ABC, automatic baseline correction), which assumes that the lowest value it measures over a period of days is approximately equal to the outdoor value. That works well in a house that is regularly ventilated, but less well in a room that never truly clears out its CO2 (for example, a room that is rarely ventilated). In that case, read the manual to see if you can recalibrate it manually.
What you can expect for your money
A reliable NDIR meter can now be found across a wide price range, from compact models to more extensive examples with data logging and app connectivity. More important than the price is simply: NDIR sensor, a clear indication of the measuring accuracy in the specifications, and preferably reviews that have compared the value against a reference meter.
What you do with it afterwards matters most
Even a meter that is not perfectly calibrated is useful as long as you use it consistently: you see trends (rises at night, drops after ventilating), and that is what matters most. Dynter does not sell CO2 meters itself, so this advice is independent and purely intended to help guide your choice.
If you want to address other indoor air issues besides CO2, such as fine dust or allergens, an air purifier with a HEPA filter such as the AirPure Plus PA244 can help with that. This will not solve a CO2 problem, that really requires ventilation. More information about the entire range can be found on dynter.eu.