I think food might be a bit of a limiting factor for getting the cost under $300, because in China a cheap meal is usually 15-20rmb or 20-25rmb if ordering takeout. So let's say you get a few baozi for breakfast 5rmb, one lunch at 20rmb and dinner takeout 25rmb = 50rmb/day budget on food, which gets you to $7/day or 210ish per month.
Actually poor people in China only eat a simple breakfast 5rmb, instant noodles (5-10rmb), and maybe make their own dinner (idk how much this would cost since I never made food for myself, but let's guess it's around 20rmb per meal).
that's $5/day which gets you to $150/month which alongside living in
>>2826380 here, which I think is a pretty ok apartment, gets you just under your $300/month rule.
You could probably get it even cheaper by living in an outlying district outside the prefecture-city urban centre, or hunting a bit harder.
and yes, I know you guys will shit on China for air pollution, but there isn't much in most of the south. I just picked some northern cities for examples since that's what I'm familiar with. Other poor provinces that you can probably find cheap stuff in:
>Anhui
>Jiangxi
>Guizhou
>Gansu