The effect is that of a cantrip Cellar. Unlike the Adventure vanilla tokens not necessarily that strong in multiples but of course you still want it on the pile of which you have most copies. I think it can get away with not being restricted to Actions as sifting during your Buy or Night phase is probably not that strong anyway.
The main reason of the Debt cost is to make this available early in Kingdoms in which it is strongest, i.e. without trashers.
And interesting idea for a "vanilla" token, but the event seems overpriced. I think +1 card, +1 action, and + are all much stronger tokens; this event should cost less than those events. It could probably be . But the cheaper it gets, the less reason you have for giving it debt cost.
A cantrip Cellar would probably be too strong for $4 and slightly weak at $5. So from this perspective it is weaker than Lab/Pathfinding yet better than Peddler/Training and Village/Lost Arts*.
The decreasing benefits of playing multiple cantrip Cellars makes it weaker relatively to the other vanilla tokens.
But Debt costs make the card available when it is strongest, i.e. early on (as opposed to getting Training / Lost Arts in the middlegame).
I don't know which of these pluses and minuses weighs heavier but as
is pretty similar to
I leave it as this price. I also think that the Adventures tokens are a bit too centralizing and don't mind if this Event is on average only bought in every second or third Kingdom.
As you pointed out, at lower costs Debt makes less sense and
or even
could be too prohibitive: in non-trasher Kingdoms you want that sifter as early as possible.
*- As Lost Arts eliminates the standard matching problem of villages and terminal draw cards (which is the very reason Village and Smithy can be cheaper than two Labs) its effect is significantly stronger than that of he corresponding vanilla card, Village, so this is where this simple comparison fails.