I'm not sure how foster care works in Japan, but I'm going to assume it's abnormal to receive a stipend there for it?
i did a brief search and it actually seems like the norm. all the search results that came up for my area (California) mention monthly payments. but (and i thought this with Marnie too at the time) it's really not like they're "getting paid" to take the child, more that it's assistance because taking care of children is fucking expensive. whatever they receive, i can't imagine they cover ALL costs of food, clothes, other expenses, etc. and even if those monthly payments did, it's still taking up your TIME. like if you're talking about net finances, there's no way you would come out positive for taking on a foster child.