I've not been able to track down these precise words to my satisfaction, have never seen, physically, on microfilm, in a period newspaper, or elsewhere." If Shane hasn't been able to track it down, the $100 is probably safe in my pocket. Martens of Montreal e-mailed recently "..Shackleton received 5000 applications as a result of a letter that ran in the London newspaper, The Times, on December 29, 1913.
The source of this is the 1923 Hugh Robert Mill biography of Shackleton [, London: Heinemann, 1923. The book, however, does not report the content of the letters nor does it mention an ad.
The funds at present at our disposal admit of the following programme.
The shore party of nine or twelve men will winter with sufficient equipment to enable three separate parties to start out in the spring.
Shackleton was not as well known prior to the Nimrod expedition and recruiting people was an issue for him at that point." Wendy Driver in London was kind enough to track down a copy of The Times article which is on page 6 of the Monday, December 29, 1913 issue: It reads: THE SOUTH POLE. I am glad now to be able to state that, through the generosity of a friend, I can announce that an expedition will start next year with the object of crossing the South Polar continent from sea to sea. Shackleton announced the Endurance expedition in a letter that ran in the December 29, 1913 paper [reproduced above] and a number of stories follow over the following weeks.