Money from Madmen in The Changeling

Since we have entered the tragedy section of the course, the amount of "trades" and commerce displayed on the stage has diminished quite a bit. Yet in The Changeling, Alibius describes his own profession--curing madmen-- as a "trade." Other passages in the play sustain the link between madness and commerce: Lollio claims that madness is a "trade to beg with"; Alibius hopes to profit from the madmen's dance. If madmen or the care of madmen are commodities, what are we to make of the social roles of Antonio and Franciscus in the madhouse? Do they become "commodities" in order to gain the best commodity, namely Isabella? If we remember the idea from 1 Honest Whore that the world itself is mad (the madhouse therefore serving as a microcosm), what is deeply ironic about Alibius locking his wife in the madhouse in order to cordon her off from temptation?