The quote from John Collier's "The Chaser" (1940) you refer to is an important one in the story because it encapsulates one of its key themes: cynicism and skepticism toward love. This viewpoint is expressed by one of the two main characters in the story—an old and mysterious shopkeeper who deals in strange magical potions:
"Oh dear, no," said the old man. "It would be no good charging that sort of price for a love potion, for example. Young people who need a love potion very seldom have five thousand dollars. Otherwise they would not need a love potion."
What the old man means by this statement is that if young people had five thousand dollars, or a lot of money, they would not need a love potion to make someone fall in love with them. Their wealth itself would be enough to entice a paramour. Thus, he puts a new, bitter spin on the idea of love, suggesting that most people treat it as a transaction, rather than a sacrament. The old man's cynicism is in contrast to the love-struck attitude of the man who's visiting his shop in search of a love potion: the youthful Alan.
Alan is desperate for an elixir to make Diana, the woman he loves, pine for him as ardently as he pines for her. Time and again, Collier highlights the radically different ways the old man and Alan view love as well as the potential effects of the love potion. For example, take this quote wherein the old man is extolling the virtues of the love potion to Alan:
"You will not have to use your imagination," said the old man. "And, by the way, since there are always sirens, if by any chance you should, later on, slip a little, you need not worry. She will forgive you, in the end. She will be terribly hurt, of course, but she will forgive you—in the end." "That will not happen," said Alan fervently. "Of course not," said the old man. "But, if it did, you need not worry. She would never divorce you."
Note the way the old man presents the love potion as a tool for manipulation. So great are its powers, Alan can use them to his advantage to get a free pass for infidelity. Of course, idealistic Alan finds the very idea preposterous for now; however, the old man's cunning tone suggests that this is inevitable.
Throughout the story, the old man also tries to pitch another potion to Alan, one which does costs five thousand dollars, which he euphemistically calls a "glove cleaner." The potion is a poison, of course—a "life-cleaner." But why would the old man champion a powerful poison to a youth looking for a love potion? The answer lies in the old man's statement below:
"I like to oblige," said the old man. "Then customers come back, later in life, when they are better off, and want more expensive things"
The sinister implication is that there will be a time in Alan's life when he will be back in the old man's shop, ostensibly for a poison for Diana to undo the effects of the love potion. His love for Diana will fade, and her ardor for him will grow oppressive. The expert salesman that he is, the old man is making sure Alan remembers the second potion: the chaser.
In drinking parlance, the chaser is the second drink you consume after a shot of hard liquor. The chaser of the title is at once the second sale the old man will make to Alan, a "life-cleaner" poison, as well as a vindication of the old man's skepticism about love. The old man also seems to be offering a warning to Alan: a love borne of enchantment or earned through less-than-ethical means can never come to a happy end.