>The excellent Seiko 5 can be had in a wide variety of styles, many for under $100. Seriously, they're very good.

There's very few (if any) Seiko 5s that are under $100 anymore. Maybe under $200.

I got myself a few Chinese-made clones of mechanical watches. They are surprisingly durable and well made, many have good movements inside (like Seiko's NHxx) and there are certainly plenty to choose from under $100.

It's a bit of a rabbit hole so be careful and not go too deep :)