$食物 : 2000
$消遺 : 2000 (tea everyday)
$水,電, 煤 : 1500
$差餉, 地租, 管理費: 0
+ 2000 rent, +600 (保險費約 + 定期覆診)
i.e. the basic expend is: 8100
As a son/daughter, you should give them some pocket money: 2000 (if possible)
I think 10k per month is 合理.
i guess she wants 5k from each of you.
for example, my mom thinks we should give her 1/3 of how much we make. but i actually give her only 5% and my brother gave her 20%. she still think it is too less from my brother. but she also have a job making 7k. don't let her know how much you make!!!!
I think you don't really have responsibility to satisfy what she needs unless you are living with her and sharing the food.
You might tell her to move to elderly home and you guys could pay all for her... it is just a few thousands per month!