Without introduction, Jacob quotes Isaiah 55:1–2: “Ho, every one that thirsteth, come ye to the waters, and he that hath no money; come ye, buy, and eat; yea, come, buy wine and milk without money and without price. Wherefore do ye spend money for that which is not bread? and your labour for that which satisfieth not? hearken diligently unto me, and eat ye that which is good, and let your soul delight itself in fatness.”
Jacob does not quote it exactly, however. One of the things that happens in the translation of King James Version verses is that places where there are italics in the King James Version have been eliminated in the Book of Mormon, and at times this occasions changes to the text. In this case, a whole section is removed and replaced with a similar idea using different words.
One of the other reasons for the change is that Joseph apparently did not understand the old use of “wherefore” as a question—where we would use “why” today. Thus Isaiah’s “wherefore do ye spend money?” could be “why do ye spend money?” In this case, the translation say “wherefore” as equivalent to “therefore” (the two terms are interchangeable most of the time in the Book of Mormon). Thus, we have “wherefore, do not spend money.“ The concept is the same, but the language changed from that in the King James Version because the use of the word “wherefore” was in a meaning not familiar to Joseph.
Of course, this suggests that Joseph somehow interacted with the King James Version when he was translating. The evidence is very strong that he did so. How it happened is much less certain.