A Reserve card that gives +1 Buy when you call it was one of Adventures' very early outtakes. It left the set almost immediately. It wasn't identical to your version, but I'm guessing it's close enough to be able to claim that it's not really worth doing. Maybe there's a way to fix it up.
Hm, do you know why it was taken out?
OMG, Settlement could have the same text as Border Village.
I mean, how often can you do that?
Oh man, nice catch! It has edge cases, but keeps it more interesting that just "Gain a Copper".
I bet Mill would be fine at $3. Maybe even $2, though then it may be too easy to clean out the pile.
We tested it at $3, but you couldn't call it for the +Buy on the same turn that you played it, but none of the other Reserve cards are worded like that (maybe, "When you discard this card from play, you may put it on your Tavern Mat?"). I made it $4 to keep it from being too similar to Market Square.