It is a common expression in England and means easily earned or easily obtained money, with the suggestion of having sold something that is normally considered worthless or of low value.
Many idiomatic English expressions seem to have their origins in the days of wind-powered seafaring.
Rope made from hemp had a limited lifetime. When it wore out it was picked apart and recycled. It was used for caulking. Rope fibres (known as oakum) were hammered into the seams between planks of a ship and hot pitch was poured over it. This was done to waterproof the ship. Of course you got money for the old rope. The phrase came to mean money for anything (seemingly) worthless.
There’s a suggestion that “money for old rope” dates back to the days of public executions in England (hangings). The hangman was responsible for his rope and was obliged by law to keep it. However, macabre souvenir hunters were willing to pay a good price for pieces of a used noose, so the hangman would cut it up into pieces and sell it, hence the phrase.
From when we were kids
THE SAYING MONEY FOR OLD ROPE IS DERIVED FROM DAYS IN THE WORKHOUSE, WORKERS WHERE GIVEN DAMAGED, AND USED ROPE TO PICK INTO STRANDS WHICH WOULD THEN BE RE SPUN INTO NEW ROPE, THEY WOULD EARN JUST ENOUGH MONEY FOR A MEAL, HENCE THE SAYING "MONEY FOR OLD ROPE"
I don't know whether there is much truth in the above. Picking apart old rope was apparently work given to people in the workhouse or in foundlings homes at the end of the 18th Century - At least according to author Bernard Cornwell - who gives the impression of having researched his subjects well.
Others say the expression is not recorded before the mid-20th Century.