That's also a pretty cool solution and it keeps more cards than mine, although I do gain the cards in descending order of cost(-ish). I'll post mine for posterity, and if anyone else comes up with a different version I'd love to see it.
Setup: -$2 token on Hunting Grounds, Estate token on a Catacombs, Watchtower and Trader in hand
Buy Border Village
Gain Border Village* ($6)
Trigger BV's on-gain, gain a Catacombs ($5)
Reveal Watchtower, trashing Catacombs
Trigger Catacombs' on-trash, gain a Hunting Grounds ($4)
Reveal Watchtower, trashing Hunting Grounds
Trigger Hunting Grounds' on-trash, would gain three Estates
Reveal Trader, gain Silver instead of one Estate ($3)
Continue to gain two Estates ($2)
Reveal Watchtower, trashing one Estate
Trigger Estate's on-trash (from Catacombs), gain a Poor House ($1)
And if you really want to, reveal another Watchtower, trashing the other Estate, to gain a Copper ($0)**.
* Hmm. I am considering the "gain the card you bought" as part of "resolving the buy". Probably didn't make that clear.
** Also hmm, I assume that the "Gain three Estates" of Hunting Grounds is three separate gains that you react to individually, because otherwise I think that gaining the Poor House would wind up covering the third Estate, meaning you can't react to it?