Yeah I wouldn't pay more than 300-400 for used, period. Otherwise I'd go new.

However used bikes can be a good option to get quality at a low price point. You have to do a lot of research to separate the wheat from the chaff, and you have to be willing to do your own work. Working on a bike isn't hard but the learning curve can be daunting.

Good rule of thumb is to expect to spend $80-100 minimum for new cables, housing, chain, and brake pads. Most likely will replace tires too, with prices all over the map. You may or may not need to replace or service the spinny bits with bearings.

That said, I've been more than happy with my 10-year-old Specialized Tricross as my gravel bike, and I've ridden over 10k miles on it so far. I bought the frame+fork used for $200 and put it together with spare parts. I probably have thousands of dollars in it but I don't keep track and that's just stuff that wears out and need replaced regardless of what bike you are riding.

Caveat: I'm slow and frequently wrong.