This is the way. As others mention opening window is your cheapest option, but you lose a lot of your heating and cooling efficiency. We installed a whole house ERV for about $10K and saw a significant drop in CO2 readings.

Whole home ones are only real ones having a chance to work.

I’ve installed mine in about 6hours (but I do have moderately accessible attic).

They are somewhat cheap too - at $1k or so.

I used to have a window mounted unit from a small company. It worked wonderfully in my bedroom.

Panasonic ones go down to 50cfm, which is fine for a bedroom. That being said, everything in construction comes down to “it depends” because there is so much variability including just in execution. In California for instance there are mechanical whole home ventilation requirements, but those requirements are quite low and are commonly ineffective for the OPs goal depending on how the system is arranged. Same thing goes for the ERV.

Often times the best way to solve the OPs problem is just to run the central fan (assuming there is central air) as that will exchange bedroom air with the rest of the house, which assuming the house is significantly larger than the bedroom, will basically solve the problem.

If you are going to do an ERV I would recommend adding a filter box for allergens/dust/etc assuming the ERV you select doesn’t have one built in. Not expensive, it’s just a sheet metal box you put filters in, HVAC guys buy and make them all the time.

If you want to spend even less money, and don’t have central air, you can install a bathroom fan in the ceiling/wall of the bedroom, and have it vent to the rest of the house but this takes a bit of thought with regards to airflow and how the air will mix. But even if all you are doing is effectively doubling the volume of your bedroom, that makes a big difference.

Agree with all these suggestions. Except I would caution against simply having a fan that sucks air out of the bedroom without also having a low-resistance path for air to enter the bedroom. If there's no easy way for air to get in, it will get sucked up through the walls and floors taking odors and mold spores with it.