Land is going to be expensive in popular areas, but dense housing means that you can get a lot more housing on the same land, so you can still contain prices, even if they're probably not going to be super low.
I would argue Eugene’s market is heavily influenced by Nike and UOregon (and recently remote worker flight from downtown Portland) and as such, drawing a line directly from its lack of density to lack of affordability strains credulity for me.
This relationship between density and cost reverses for nearby towns like Springfield even though they share many of Eugene’s characteristics.
Let's look at the prices in Chicago, which is a pretty dense place: https://www.realtor.com/realestateandhomes-search/Chicago_IL... - $350K
Now let's look at the fairly bland, suburban college town where I grew up, which is not dense: https://www.realtor.com/realestateandhomes-search/Eugene_OR/... - $495K!
Eugene is far more expensive than the dense place with a lot of amenities and jobs.