I have Thai friends who live and work in Bangkok. The locals generally pay between approx. 4,000 - 5,000 bath a month for 2-3 star apartments ($US 160). Then you add internet, power, cable, phone, etc, gets up to around 7,000 bath a month ($US 225). Then you've got your food, other expenses, etc, may be 2,000 bath a month ($65). But that's if you spend very minimally (a decent meal for 2 at a restaurant in the big department store (MK Suki, Sukishi, etc) can set you back 700 - 1,000 bath easily). So you would have to be careful where you eat too.
So yeah you can live off of $300. But it'll be in an average apartment, in an average neighborhood, etc. You'll also have to watch how you eat, how you spend your money too. If you have a lived in GF either you or her would definitely have to get a job to pay for anything extra.
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If you're looking at a proper condominium with swimming pool, a gym, in a swanky part of town, you're looking at 15,000 bath per month minimum.. which is approx $US 480.. usually well off Thais stay in these Condos.. they buy them outright.. either that or Ex pats living and working in Bangkok who can afford them...