I don''t think I've spent over $400 yet on a bike except when including s/h on one and lots of upgrades on another, and my Paramount which has inflated price due to it being more collectible and sought after I guess.
All under $400 (avg price around $200)
This old Voyageur I even fully trust and enjoy on fully loaded trips/tours, though racks and bags put it over $400 the bike itself was far less:
You get the idea. These are all fine riding bikes. And you can upgrade them with modern components too if you like.
Here is one I upgraded to mostly all new and newer components. I bought the bike complete and rideable for $100 and change, but stripped it to frame and put all new stuff on it. It has over $400 into it now (used wheels were almost that much alone) but just goes to show even a cheap bike can be made nice as modern bikes and you can take your time and split up the cost over a while (I collected parts for over a year until I was ready for the upgrade).