Who is Jacob? And where did he say this? In Karl Marx "Das Kapital", in Ch I.1 we find this quote 


  "Jacob questions whether gold has ever been paid for at its full
  value."


Is Jacob Vanderlint? Where in his book “Money Answers all Things” is that claim about gold's value?
As we discussed in the comments, in  William Jacob's An Historical Inquiry Into the Production, and Consumption of the Precious Metals, on page 101 it says


  It is probable that in all ages those metals have cost more in their
  production than their value ever repaid.


This is the likely source of the quote.