There are several papers out there with many numbers to consider, but I'll point to a recent one from a paper by Michael Baker, Jonathan Gruber and Kevin Milligan. They looked at women whose children were born when Quebec first introduced their larger program for affordable care.
Over the course of their lifetime, they find themselves with higher wages and better occupations. That effect lasts much longer than just the time when you have a small child. They found the fiscal return to be somewhere between 75% and 117% of the initial expenditure.