Julian Savulescu suggests extending the idea of paying drug addicts not to have children to everyone.  At first the purpose is to avoid the eugenics feel of discouraging only one set of people from procreating, but then he reasons:

“The benefit of a policy of offering inducements to sterilisation is that it would select those who do not value, do not understand, do not want the role of parent. And it is precisely these people who are likely to be the worst parents.

Being a parent is, at best, a difficult job. Why not excuse those with the least motivation and determination? There are plenty of others willing to take their place. And the earth can only sustain a finite number of people.”

It’s of course true that if you penalize an activity, those to whom it is most expensive already will be the ones to quit. However:

1. The existing costs of parenting already induce those who dislike parenting most not to parent. Adding another cost to parenting would just move the line where it becomes worthwhile to parent, not implement such selection. Justifying this requires an argument that the level of value at which people find parenting worthwhile is too low, not just a desire to encourage better parents to do a greater proportion of parenting in general.
2. “Excuse those with the least motivation and determination”? We aren’t exactly pushing them to do it. Why presume they don’t excuse themselves at the appropriate point? This goes with the above point; the line where parenting seems worthwhile could be in the wrong place if parents were pushed for some reason to have too many children, but why think they misjudge?
3. Why would there be plenty of others willing to take their places? Presumably those wanting to bear children will do so already or at least would not start at a 1:1 ratio on the news that others are not. Few factors influencing conception depend on the ambient birthrate.
4. If others really were willing to ‘take their place’, the exit of poor parents from parenting  wouldn’t be relevant to the total population and whether the planet can sustain it.
5. Presumably the issue is how big a finite number of people the earth’s resources can support, and more importantly why and to what extent parents should be expected to misjudge.
6. Smaller populations are not automatically better if you value human life at all. That parents are unlikely to account for the entire value of their potential child’s life is a strong reason to think that parents don’t have enough children. If that is the overwhelming externality, the line should be lower, and we would be better off paying people to have children.