I've tried a handful of used hikvision / dahua in the ~100usd range but they don't play nice with anything other than the vendor's software. Like VLC / open source VMSs can't play the RTSP stream. And even then, its a crapshoot whether features like stream configuartion or PTZ work.
I know the ONVIF group publishes a list of verified-compliant models but it's been a pain trying to find one that's affordable and in stock.
I've heard from devs that work on this stuff that most of this pain stems from how loose the ONVIF spec is. Too many optional features (even per "profile"), too vague on requirements, leading to lots of vendor-specific metadata / camera quirks.