Ihave already computed the charge of nursing a beggar's child (in which list i reckon all cottagers, labourers, and four-fifths of the farmers) to be about two shillings per annum, rags included. . what is swift making a satirical statement about?

Swift is making a satirical statement of several concepts.

One would be on the objectifying of humans (computed cost for babies...)
Two would be the idea of how they can easily spare the children by merely reducing them to costs, especially from poor parents.

I. can’t see the paragraph