In my considerable daycare research - near me BY FAR the least expensive option is an in-home babysitter (who does it at her own home). It can be about half as expensive as a day care center. Again, this varies by region - but near me you can find good in home sitters for $25 per day per child (with a slight discount for weekly and maybe discount for 4 kids - I only have 2 so I have no idea). So I would guess unless your area is vastly different than mine - that you should expect at LEAST $75 per day for good day care - that's $375 per week. After you add in your transportation to/from work costs AND any $ on other work-related things (new clothes, lunches out) - I would say you want to make at least $550 or so TAKE HOME (after taxes) in order to make it close to worth while (unless you get a lot of good health care benefits or something - that might factor in to make it worth it to take less take home $).
Of course, maybe my calculations are way off for your area. But I would start by asking everyone you know who is in your area what they are paying (if they don't mind you asking - most won't if you say you are researching getting some sort of child care) and WHERE they go (day care center, in home, etc) - that way you'll have a real basis to begin to compute the numbers and what is necessary for you to make to make it worth while. And THEN - find out what you COULD make.