Human life should not come down to economics. You cannot put an economic value on human life. Any attempt to do so is inhumane and barbaric.
Also, treating abortion this way is tantamount to eugenics and social engineering. Poor people's babies have the same right to life as those of the wealthy. Permitting abortion based on the notion that poor people get more abortions and their children cost the state more is a form of sociological engineering, particularly given that ethnic minorities are overrepresented among the working classes.
Finally, the notion that someone from a poor background is likely to be a net recipient from the state is just not true. The average child born into poverty goes on to become a net contributor over the course of their lifetime. In fact, less than 5% of children born into families that receive welfare payments will remain on them as adults. Therefore, abortion does not even make economic sense.