Much much easier if you can show a compelling reason for your gf to return to Thailand - A business, a,job, owning property, kids or other responsibilities. I did my partner's ..straightforward as she was in employment.
VERY subjective question and will attract different responses based on people's lifestyles. I could quite easily live comfortably for my needs on that amount and even less. I don't smoke or drink anymore and I am no longer interested in eating at fancy restaurants, or one for glitz and glamour.