If it's truly 3/8", then the glue-down hardwood is half the thickness of a traditional hardwood flooring material (example - 3/4" thick oak). This uses less volume of wood per square foot of floor, hence the lower cost. The boards may also be narrower, which makes them cheaper to produce (per sqft) than wider boards.
The downsides of the thinner material are that a rigid subfloor is required, along with glue rather than nailed installation (nails would risk splitting thinner tongues). And there may be less material you can sand/refinish in the future, though I think that would be somewhat mitigated by the fact that you don't have any nails in it.
Finally, I don't know how glued hardwood performs long-term, but I'd be a little concerned about the adhesive failing after years of the wood expanding and contracting. This is not a problem for engineered woods because their engineered lumber base is stable, but traditional hardwood floors usually have some allowances for expansion.