There’s not much below $500. Here’s what I could find:
Another option is to buy a Chromebook, or look for a second hand several generation old Lenovo Thinkpad Carbon X or Dell XPS Developer Edition. The latter is your best bet for not getting something underpowered, but also carries more risk of it breaking down sooner with no support possible. You might be able to find a first gen framework 13 second hand which can be fixed if something goes wrong, but it hasn’t been around so long that they are that cheap. Still someone might want to get rid of it and low ball it.