I'd guess 10,000 Baht would be enough to live comfortably in most parts of Isarn, barring in mind the fact she'll be getting easy money will mean other family and friends looking to her to feed their habits and hunger - and maybe in time encouraging her to push for more money and 'emergency loans'.
IMO, the amount is usually irrelevant with regards to whether someone will maintain a drug habit, freelance on the streets, or sleep around in general. If those habits are in their make-up, pre-existing, or they are hanging around in the wrong crowds, their budget won't make make too much difference. There are ladyboys out in the boondocks earning 50 baht per day, or even in big cities, working the early morning 7/Eleven shift, who work hard and have a lot of self respect, while there are others out there getting 'sponsorship' money of 100,000+ per month, promising their farang boyfriends they are behaving while they still freelance for small money, party with their friends, and even pay to sleep with karaoke boys.
I've never 'sponsored' someone but I'm a little less cynical about it, and I don't see the problem when guys earning big money in the west send a little back to a favourite girl or ladyboy, I just don't like to see it done by guys who can't afford it, or the guys who do it hoping they'll get more love and loyalty in return. If you want to help someone and you have money to burn just offer what you can comfortably afford and if they are a decent person and have a connection with you they'll be grateful whatever the figure.