If you are buying for the two of you rather than for someone else: dice or a tumbling game, not a prompt deck. The whole point is that neither of you has to be the one who suggests something. A mechanic does the suggesting, and that is what makes it easier to start on a night when nobody feels like starting.
If it is for a bachelorette: buy one game with a mechanic and fill the rest with small things. One tower or dice game carries a whole night. Pins, candy and keychains do the gift bag.
If they already own a shelf of games: go for a book instead. A position or bondage guide gets picked up more than once, and it does not need two people in the same mood at the same moment to be worth anything.
If you have no idea what they are into: plush. A bondage teddy is funny before it is anything else, it survives being opened in a room full of people, and nobody has to explain it.
On budget: the small things punch above their price here. A pin, a keychain or a candy costs a few dollars and lands better in a pile than one expensive thing in a box.
