The quote in question appears fairly early on in the story. It comes right after the old man explains that the poison he sells is quite untraceable and quite expensive. He says that his "life-cleaner" is a five thousand dollar purchase, and Alan quickly explains that he wants nothing of the sort. Alan also states that he hopes that all of the old man's potions are not so expensive. The old man confirms that not all of his potions are extremely expensive, and he claims that they do not all need to be ridiculously expensive. In fact, he sells his love potion quite cheaply, and he gives a quite cynical reason.
"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."
The old man says that wealthy men looking for love have no need of a potion because they have money. He might mean that wealthy men can buy love, or that wealthy men attract women because of the wealth. The old man is saying that love is easily found for the wealthy because of the wealth, and poor men need the love potion because poverty isn't likely to attract a lover; therefore, there is need of chemical assistance to obtain love.