This is extremely good. Better than Counterfeit for trashing Coppers (barring the +Buy), same if trashing Silver. And Counterfeit is a pretty good $5. This should probably cost $6 or higher. At $6 may be ok, because is a bit late for trashing, and as you said, it has some issues if there is nothing you would like to trash. It would be way better than Gold for engines, but not as good as Counterfeit for those, because $5 is much easier to get than $6 and Counterfeit comes with +Buy, and for BM-ish it is probably on par with Gold, as trashing Copper is not as good for BM. For BM with Copper haters (Venture, Adventurer), this could be great, and also in Colony games when you want to trash the Silvers as well.
I would just make the trashing mandatory. You can always not play it, and there are no accountability issues.