I think that in almost every game with farmland, if I draw a $9 hand, I will farmland a gold into a farmland. The reason is that a farmland in your hand is worth $2 because it makes a province only cost six. So think of it as turning your silver or gold into a harem... which seems like a good trade any time you'd want a province anyways. Of course, if you draw $5+farmland, it is bad, so you have to decide how consistently your deck is going to hit the six needed.
If you have a way of generating gold, such as tunnel, it seems even better.
Similarly, on the topic of buying farmland over gold, I would say its also usually worth whenever you are to the point of buying provinces somewhat consistently. This is especially true when you can remodel into a key component.
As with anything, farmland tricks get more attractive if you are behind. If you need extra points to win, you need extra points to win.
I would consider this embargo trick on the board suggested, because jack gives you lots of money and gets rid of your estates. However, on some other BM boards (say smithy or margrave), I think you risk hitting 8 less consistently if you pull this trick early. I would probably buy 2 golds before I did this.