"double jack" is pretty formulaic. Your buy priorities are, in order:
1. Province
2. Duchy/Estate if game is close enough to ending
3. Gold
4. Jack (up to 2)
5. Silver
That's it. It is a pretty fast strategy and doesn't require much thinking, but it's rarely the best strategy on any given board. In def's example, there are no $5 cards, but usually getting any decent $5 card is better than a second jack. In the example specifically, I think double jack should lose to courtyard+money or jack+warehouse.