The average we found for institutional care for infants on the eastside and northside (looking at about 30 places) was apx 1600 per month.
In our region infant care is most expensive, and then the price starts dropping dramatically only after potty training.
Good nannies (my favorite option) range from about ###-###-#### or 1500-2500 depending on hours/days/times/ages/& whether or not you're doing nanny-share.
Au pairs (if you've got an extra bedroom) through the US State Department cost about 4000 up front (paid to the agency which is held in reserve) and then 150 p/week + room & board. There are different programs through the State Dept. Students can only work 32 hours p/week, non students 40, etc. So the monthly cost if you distribute everything is about 933...but you DO have someone living with you. While I know 7 or 8 families that have au pairs, they are the most popular option I know of for single parents.
Anyhow...hope this helps. :P I personally had to quite my job when my son was a baby and go back to school, because I quite literally could not afford to work.