Simple Version
Buy Priority AKA Your "shopping list"[/u]
Are there Junk attacks? If so, buy 2.
Are there cards that give +3 or more cards? If so, buy 2 and stop here.
Are there discard attacks? If so, buy 1.
Are there cards that give +2 or more cards? If so, buy 2 and stop here.
Are there cantrip trashers? If so, buy 2.
Heuristics/Buy Priority:
1. Buy the first two items off of your shopping list as soon as possible.
2. Can you afford a Province? If so, buy it.
3. Are there 5 or less Provinces? If so, do you have more non-Province VP cards than your opponent? If not, buy a Duchy.
4. Are there 3 or less Provinces? If so, do you have more non-Province VP cards than your opponent? If not, buy an Estate.
5. Can you afford a gold? If so, buy it.
6. Is it after turn 5? Buy a third action.
7. If it after turn 10? Buy a fourth action.
8. Buy silver.
9. Buy nothing.
Complicated Version
Economy
Silver: Very rarely do you need more than 5 silver in your deck.
Terminal Actions: Buy no more than 3 in the first 10 turns.
Early Terminal Collision: If you draw a terminal, there is a ~40% chance of it will collide with another terminal.
Buying at least one 5 coin card by turn 5: Silver/Silver*= >99% chance. Silver/x = 70% chance.
Buying multiple 5 coin card by turn 5: Silver/Silver= 55%. Silver/x = 10% chance.
Buying 5-coin cards by turn 4**: Silver/Silver= 90% (15% of being able to buy 2). Silver/x = 50%. x/x = 5%
Provinces: Don't buy Province #1 until you have 2 "functional gold".***
Actions
+Cards: +1 card = +1 coin or +1.25 or +1.5 coins (early, mid and late game, respectively)
Trashing Rules****: If you have a trasher in hand, here are your priorities: 1:Trashing two+ Estates, 2: Trashing three+ copper, 3: Buying an extremely important card (Witch, Mountebank, etc.), 4:Trashing 1 Estate, 5:Trashing 2 Copper.
Curse/Junk: 1 junk card = opponent wins 1 turn slower
I'll explain the statistics behind these in a bit. I've got a few more for evaluating engines.
[edits]
*- This is specifically referring to opening Silver/X or Silver/Silver
**- Updated to include Turn 4 probabilities.
***- The "advanced" version states, don't buy a Province in slog/BM if it will drop your money density below $6/5c. An example of a "Functional Gold" includes cards like Smithy (eg. if your money density is $1.25/c, a Smithy on average is worth $3.75)
****- Updated the "trashing" section to further clarify. The reason trashing 3 coppers is > than "buying extremely important card" is that, if the card is so crucial to grab that it's worth holding on to 3 copper, then it would be smarter for you to just open silver/silver, and pick up your trasher on turn 3/4.