And where do you get a modern road or gravel bike for $600 or less? Even used ones are often more expensive.

https://www.craigslist.org/view/d/spokane-univega-modovincere-campy/2fCfoDh8GyaNnA5TYiUNLM

https://www.craigslist.org/view/d/spokane-trek-560-road-bike/6ZBGHtX6aNZuBCiUW6KurX

https://www.craigslist.org/view/d/four-lakes-quintana-roo-caliente-road/jpKUvx71aicwA3zwaKRa8F

https://www.craigslist.org/view/d/post-falls-womens-bike-carbon-and-crazy/1TUjxPdBWzKLZEvpezwNWg

https://www.craigslist.org/view/d/coeur-alene-raleigh-road-seven-hundred/18gRozwJkg2gjHQEfBkCY5

https://www.craigslist.org/view/d/coeur-alene-new-updated-ozark-trail-g1/sktDtYv7oX92FMLCEEb3Lz

Some of these might not be as "modern" as you'd like, but they'll work just fine anyway.