Hypnodyne Zmax is a EEG tracker The Quantified Scientist Rob ter Horst uses as a reference device for sleep accuracy testing:
Costs for Hypnodyne Zmax: 1000 – 4000 USD
Costs for Dreem 3: (ask for a quote)
Rob ter Horsts opinions are completely his own. However, he does earn a small commission if you purchase any of the reviewed devices via his affiliate links.