There's a bunch of cultural reasons for the low birthrate but a bunch of encouraging benefits might help address that.
While it might be true that everyone has a price, if we plucked someone off the street and they told us they never wanted children (at all, or more than they have now), how much money do you think it would take to persuade them to have a kid?
Do you think it would be $1000? How about $4500? Maybe it costs a whole $12,000 right? These are the sorts of incentives that are offered in Europe, in South Korea, etc. They don't seem to influence much extra in the way of births. And it's not difficult to see why... those people are told (whether true or not) that children are far more costly than those sums. So we're still talking about it being net negative.
In some publications, people in the western world are told that it's some large fraction of a million dollars to raise a child to adulthood. How many babies could Japan afford, if it had to pay parents $500k for each?
It's even worse than that though. Many Japanese women of child-bearing age aren't even in circumstances where it is plausible for them to consider having a child. No husband, or a husband whose career doesn't make being the sole provider possible. Little chance of those circumstances changing before motherhood is out of the question. Etc.