What does the phrase 'infinite good is being bought' (lines 21 - 22) mean?

Answer this question based on the following passage.

            The treasure of wisdom and science, which all men desire by
            an instinct of nature, infinitely surpasses all the riches of the
            world; in respect of which precious stones are worthless; in
            comparison with which silver is clay and pure gold is just a
(5)        little sand; at whose splendor the sun and the moon are
            dark; compared with whose marvelous sweetness honey is
            bitter to the taste. In books I find the dead as if they were alive;
            in books I foresee things to come; in books warlike affairs are
            set forth; from books come forth the laws of peace. We must
(10)      consider what pleasantness of teaching there is in books,
            how easy, how secret! How safely we lay bare the poverty
            of human ignorance to books without feeling any shame! They
            are masters who instruct us without rod or ferule, without angry
            words, without money. The value of books is unspeakable; no
(15)      dearness of price ought to hinder a man from the buying of
            books, if he has the money that is demanded for them, unless
            it be to withstand the malice of the seller or to await a more
            favorable opportunity of buying. For if it is wisdom only
            that makes the price of books, which is an infinite treasure
(20)      to mankind, and if the value of books is immeasurable, how
            shall the bargain be shown to be dear where an infinite good
            is being bought?