You're probably right, bang for your buck wise.
You can't really beat a fresh & constant incoming air supply though, right?! HRV/ERVs (depending on your climate, humidity, etc) are proven to lower the levels of pretty much anything and everything that could be floating around, or even off gassing (lol) or whatever inside your home. Including PM spores, and a host of other allergens & stuff that build up with stale/recycled air. Well designed units are good at recapturing a huge amount of the heat (or even the cold AC air, and vice versa) that would be lost if venting without any recovery unit, making it a very worthwhile investment towards clean air and overall quality of life! As far as using them for growroom/greenhouses, there isn't much info out there though. Lots of DIY guides though, which is cool.
While they are technically designed to lower c02 levels in a tightly sealed home, they still keep supplying it at all times, at the natural healthy levels at least. Just like being outside. Probably around the same 420? or whatever level, but for free without needing a bottle
Well, not really free though, because the commercial units are quite expensive to initially buy and install, and do require maintenance throughout the year. Like installing new filters/cleanings every few months. I guess you could use real HEPA filters circulating the air in a c02 suplemented sealed room instead to clean it up, but that would probably be even costlier to setup/upkeep, and still not as good overall as the fresh high quality vented in heat recovered\filtered air from outside coming in at all times..