Here's a start strategy, which has many possible improvements. Breaks no buying anything more than $2, but it is Saboteur-proof. I'm not going to bother figuring out average turns right now. Maybe later. It might not even be valid for this puzzle anyways.
Open Chapel/Secret Chamber. Chapel down to a 4 card deck: Chapel, Secret Chamber, 2 Copper
Secret Chamber -> buy Silver (CH, SC, cop, cop, sil)
Chapel a Copper, then buy Silver (CH, SC, cop, sil, sil)
Chapel Secret Chamber, buy Bishop (CH, Bishop, cop, sil sil)
Bishop Chapel, buy Gold (Bishop, cop, sil, sil, Gold)
Now you can do a 5-card Bishop trashing Province bonanza and they can't do anything about it. HAHAHAHAHA
Also Secret Chamber opens up some possibilities involving decks with more than 5 cards. Use the reaction to put cards that won't be hit on top, saving the cards you want. It would only be guaranteed to work on 6 card decks though. Otherwise you could have the 2 cards in your deck be Secret Chamber and the card you want to protect.