What about, "You may buy any number of cards costing more than $0 this turn"?
I think this would be reasonable (though very situational) at $2 or $3; it's rarely better than Squire. (Edit: Ninjaed.)
For mail-mi's original card, I would try it at $6 or $7, and ban Gardens, Goons, Bridge and Highway. That price should prevent it from being an automatic buy in most games - you could usually buy a Gold or even a Province instead, which help you to win the game, not just give the option to end it one shuffle later. It does completely mess up the endgame, but I think that could be interesting. When there's no cursers or looters, the card effectively allows you to:
(i) end the game on 2 empty piles (by buying all Coppers), or
(ii) end the game on 1 empty pile when leading by >10 VP (by buying all Coppers and Curses)
This would shorten the game notably, but should not allow the Duchy Rush to be a viable strategy in most games. However, with cursers or looters (for an extra $0 pile) in the game, it might be too dominant.