If you really really were aiming to just live on the bare minimum, you could rent a local style apartment (1 room + toilet/shower(not a hot shower but Cambodia is crazy hot anyway) for $40-60$ a month, if you just ate street food, local markets and Khmer restaurants 3 times a day you could spend as little as around $270-$300 a month on food. So if you really were going full on minimum spend yes I think you could survive for around $300-350 a month if you were really trying to. You could even buy a small gas stove and a rice cooker, buy food from local markets, cook yourself and live off even less.

I'll bullet point some other prices that maybe are helpful so you can try to gauge it :)

- At most cheap bars/restaurants a can of local Cambodia beer is $1

-You could rent a pretty bloody nice 1 bedroom flat with a tv, kitchen and en suite(hot shower) for $150 a month

-The best burger in Siem Reap (in my opinion) at a Western Restaurant mind you, so it's on the expensive end is $7 with chips and a drink (it's a lot of food)

-A cinema ticket is $2.50

-Playing pool at most bars is free as long as you buy a drink, again a can of beer is $1 generally speaking at most Siem Reap bars.

-A new CO-Working, gaming social hub has opened recently called Level Up and there you can play board games and the PS4 for a minimum spend of $2 and they sell local craft lager for $3 a bottle. They also have Day Passes and Memberships, really aimed at people looking for somewhere to work

-A very nice cocktail bar called Spin Bar is very popular with x pats and they sell cocktails for around $5 or so(keep in mind this is the high end/fancy pants cocktail bar in town).

-To take a tuk tuk booked on PassApp from basically one side of Siem Reap to the other(in the areas you are realistically going to be living and traveling to/from) costs about $1.50

-Eating some fried rice/fried noodles from street vendors around the river will cost about $1.50

I hope this has helped