How much net income would it take for you to keep working vs. being a SAHM?

A friend of mine works full time and has her child in daycare. After she pays her daycare bill every month, her net take home pay is only $300. So essentially, she is making $.19/hour instead of staying home.
I know there are a lot of other reasons to work besides income, but generally speaking, is there a dollar amount you would need to earn in order to keep working vs. stay at home?