A meaningful amount, like $400~$500 a month could make a much more considerable impact. $100/month means "Maybe we can take the kids to a movie one a month or have a nice dinner to help distract ourselves."
$500/month could mean, "We can get all those extra school supplies or put our kids in extra school programs."
Seeing if people would make those decisions would be an equally important study.
These current programs see more designed to encourage people to use free programs with a small monitary incentive. I probably says more about those programs than the impact of the money itself.