I'm intrigued by the $5 or Mystery Money idea, as it sets up an interesting conundrum of sorts. If I'm only down by one or two questions, then sure, take the $5 and get myself closer, esp. if the Speed Round is coming up. If I'm playing for the Lot or a big cash jackpot, then yes, take the money card and pad my lead/reduce my deficit.
If it's a runaway, then yeah, I prolly take the Mystery Money, knowing I can go home with a few hundred bucks in my pocket, win or lose. Honestly, the sure thing with the Mystery Money would make me lean towards the latter, regardless of my score.