Both Winner's Board and WBMG feel, to me, awkwardly tacked onto SALE; "Let's go shopping" is how the game should end.
But if you want an endgame, and you need to use some elements of the NBC show, why not base it on the Fame Game. Let's put five clue cards behind the numbers, three money cards (say, $500, 1000, and 2500--tying into the main game version), and one Stop sign. (Hey, the spaces were already octagonal, why not use it?)
Pick a number. Find a clue card, Jim reads the clue, you try to solve. Don't know the answer, try again. Pick a money card, that's what you win. Pick a stop sign, game over. Solve the puzzle, win today's prize, whatever that is.
All that said, I think TEMPTATION (the Aussie version) did the endgame as well as it could be done, and still feel like part of SALE: a series of questions that add to the champion's prize fund.
Of course, to make either of these ideas work for the NBC run we'd need some pretty sophisticated time-travel equipment.