$2.99
You clearly count and physically show $30 during a clever riddle. Within the story of the riddle, $1 goes missing, yet everything still adds up correctly.
Meanwhile, magically, $1 visually goes missing as well! You recount the money and sure enough the dollar is missing! You will get a 12 page PDF with very easy instructions on how to make this gimmick in seconds.
This is a perfect combination of magic, puzzle and riddle. Does not require difficult sleight of hand. The money does the trick for you.