very hard to say because it depends totally on
A) How much the girl wants to be with you
B) What dependants the girl has (Kids / Sick mother / Thai Boyfriend)
C) How long she has been in the scene and if she likes it
D) How much of a stunner she is and how much she makes now
E) How well you treat her as a GF
F) Do you expect and give fidelity !!
Lets say that at a minimum she will need to cover costs (keeping her own room, basic life etc) and does not move in with you permantly (keep the room.. its worth it.. I PROMISE you) you would be needing to look at min of 8 - 10k per month.
As a more normal figure so she can send some money to momma, get some clothes and shoes, keep some face and have some money in her pocket to buy her mates the odd drink, if your not 25 with a 6 pack (not the carry out kind) 15k per month is normal and not really abusing you.
This is assuming they are not moved in totally and permantly and so have a running room cost of (guesswork shared room and rented TV) of 3k or so... Maybe renting a motorbike or other fixed costs. Numbers would be adjusted if she has none of these fixed costs or you take care of them directly and she becomes a full time live in girlfriend / wife with no room. Personally the room is the safety net and one that I advise should be kept for many months. Also once in a living together setup how much is for her and how much is housekeeping, does she pay off any bills or go to the market etc on money you have already given her ? Its easy to forget that she spends money on you if you dont see it, try to spot this and treat her accordingly.
You must also remember a absolute stunner may have 3 or 4 buffalo farang each sending 20k per month or so.. I have known some girls on 100k baht before they even worked a night simply cause they were good at finding fools and spinning thier magic on them. A girl like this who has been in the scene for a while is a much harder traget to pin a cost on, are they being honest with you, do they have other sponsers, does she enjoy her time with you or is she working you. All those are tough questions that dont get easy (or honest) answers not only from her but from yourself, look in the mirror and ask does she like me for me or does she like me for my money or the lifestyle I am giving her ?? Now wonder if you believe the answer
I think this factor alone is why many of the long termers / expats will be looking for a less hardened girl for long term relationships, they will expect less and give more and leave you in less personal doubt.
Any girl that 'demands' money to stay with you should be axed from the list.. Shows a mentaility to drain you from the outset and starts the relationship badly. Theres plenty of sweet girls that will work away at you without demanding anything directly.
In a long term relationship I think its better to get the money thing sorted and clear.. Then have that allowance dealt out (though not in one chunk as the month will be spent in a week) and clear that thats all, not an allowance and drip drain system also. Easy to say and tough to do.
Any hocking of jewellry and or phones etc stay hocked until the next pocket money date etc. No need to be hard or cruel just firm and clear, many of these girls need a little education in fiscal responsibilty and getting them to live in a non hand to mouth way and budgeting is actually good for them (I know that sounds patronizing as hell but it is true)..
This conversation and topic can be discussed after a couple of weeks to a month if things are going well.
Damn that was much more than I intended to write... Complete financial HOWTO there...