一本一利 Capital and Profit Alike
行者搗鬼道：「我們原來的本身是五千兩，前者馬賣了三千兩，如今兩搭聯裡現有四千兩，這一群馬還賣他三千兩，也有一本一利。夠了，夠了。」八戒要睡的人，那裡答對。行者捣鬼道：「我们原来的本身是五千两，前者马卖了三千两，如今两搭联里现有四千两，这一群马还卖他三千两，也有一本一利。够了，够了。」八戒要睡的人，那里答对。
逐句解 Line by line
Pilgrim, playing the mischief-maker, said: "Our capital to start with was five thousand taels; the horses we sold before fetched three thousand; there are four thousand in the two saddlebags now; and this drove will fetch another three — capital and the same again in profit.
搗鬼 is to play tricks, to be up to something — the narrator naming the act as he reports it. 兩 is the tael, the standard weight of silver. And a small thing worth hearing: in 一本一利 the two instances of the same character take different tones, yì before the level-toned capital and yí before the falling-toned profit, because a yī shifts against whatever follows it.
Enough, enough."
Four characters of feigned satisfaction, pitched to carry.
Bajie, being a man who wanted his sleep, was in no state to answer.
那 is nǎ, the rhetorical one — how would he answer, meaning he did not. 答對 is to answer back, to make a reply.
Pilgrim is talking to nobody. Bajie will not answer and the other two are asleep. The speech has one purpose, which is to be overheard.
The figures are worth adding up because he means them to be. Five thousand in capital, three thousand from the last sale, four thousand in the bags right now, three thousand more to come. It is a fortune announced at volume, in a yard, in the dark, by a man who has already noticed that a dozen farmhands are sleeping within earshot.
The chapter has spent two thousand characters getting four monks safely invisible, and Pilgrim throws it away out of boredom in three sentences.