Savings
A copper that, if not initially spent, is retained on your Reserve mat until such a time as you need it. I initially worded at beginning of your Buy phase, but I could probably loosen that. My wife and I tried it and often forgot to call it until other Treasures were already played. It never hurts to call it every Buy phase though, because if unspent it returns to the Tavern mat. Note that this cannot be called in the Action phase for, say, Black Market.
Just realized something broken about this one. As worded, you only need $1 unspent to put them all on your Tavern mat, no matter how many you have, effectively making all but the first one Coin Hirelings, which would probably cost $6! As it is now, I'd say it's way too strong for $1.
Based on what you said in your post, I'm guessing that your intention was that you would need $2 unspent to put two on top, $3 to put 3 on top, etc. In that case, you'll just need to change the wording (I know my first paragraph is true because of the rulebook explanation for Wine Merchant). Here's my suggestion:
Savings:
$1
When you play this, you may put this on your Tavern mat. If you do, this is worth $0.
At the start of your buy phase, you may call this, to play it.