If then it is for a single person who can spend with caution, then around $150-$200 is okay which will be around 45,000 naira - 70,000 naira in my country currency if i am right with the conversion rate. $200 should be pretry okay for a dependent person but just like you said, it is still high if converting it to steem as it will be equal 500-600 steem also.
But also if steem rise in value and hit like 3$, then the amount of steem needed also will reduce compare to the current price so i will say it falls into three ways
One, how the person spend or how the person budget his spending per month.
Second, the area or the city the person live at. Like you said, london is pretty expensive living compare to south africa also.
Thirdly, the value of steem price as if steem is worth more, then the steem require will be few but if steem is worth less like this current price market, then more steem will be needed.
This is my view on it