According to a survey done last year, you are "white collar" (upper class) if you earn in excess of 3000 RMB per month in Beijing. Of course, keeping you collar white here is pretty difficult.
Personally, I don't see how anyone can enjoy a quality life here on less than 4000 with housing included. That still means eating a lot of meals at home, but at least you can afford to go out every weekend. Without housing...yikes.
As far as how much money a visitor needs to live here for a month...that also really depends on a lot of factors. Are they staying with a friend or at a hotel? Staying a month in a hotel will set you back well over 4000. On the other hand, you can get dorm
beds for 60 a night, which adds up to 1800 over 1 month.
Food-depends on how you plan on eating. If you plan on staying with friends you may just need to kick in for the grocery bill once in awhile. But if you are eating out every night, the expense can be very varied. You can live off chuan, gai fan and noodles
for 10 kuai a day...or you can drop a few hundred a day at pricey restaurants. I'd guess a daily food budget at an average of 50 kuai per day...which brings the toal to 1500.
Tourist sites. Are they going sightseeing? That can get expensive. The average admission price for most Beijing sights is about 100 kuai...and that's not including transportation. Including transportation budget another 800 to see sights.
Are they a shopper? A lot of people like to shop at places like the Silk Market, Wangfu Jing and so on. Even if you have really good bargaining skills and great Chinese, this can get get expensive. I hate shopping and I have still managed to drop 500 kuai
at the silk market...bargaining HARD. If you are a die hard shopper...double this number.
Bars. Does the person in question like to hit the bars? If so, why and what for? A lot of people go to bars and don't drink. But if you do drink, going to the bars will add up. If you're drinking, budget a minimum of 150 for a night at the bar...more if
you're going to a club. Clubs are ridiculously expensive.
So....to sum up I'd say bring 10,000, a bank card and a credit card. Probably won't need more than the 10,000, but better safe than sorry.