In Beijing the average price is between 2000-4000RMB a month, give or take. Expats tend to overpay, Chinese employers are not as generous. You don't have to pay the inflated laowai price though, just be firm.
There are plenty of agencies (marketed to locals) in every city that have loads of ayis. Sometimes that is actually a better way to get a cheaper ayi, plus they sign contracts with their agency so usually they won't just take off at moment's notice and if they do the agency will send someone new. Their prices are targetting locals so if you tell them you don't want to pay more than X they will usually stick to that. There are also ayi agencies that specialize in ayis for foreigners that cost an arm and a leg so beware of that.