1. Conspirator: $7 cards for $4.
2. Wishing Well: Laboratory for $3.
3. Tournament: Gain a Prize!?
4. Treasure Map: Gain four top-decked Golds!?
5. City: Laboratory + Village + Market!?
6. Gardens/Vineyard/Silk Road/Duke: Play them right, and they're better than Provinces for cheap.
7. King's Court: Turn one Mountebank/Grand Market/Rabble into three.
8. Adventurer: Terminal +$6. Platinum games can be +$10.
9. Counting House: Terminal +$7 out of the box with no hard upper limit.
All these cards have can be far worse when conditions are not met, though; most can be worse than a cantrip.
1. terminal silver
2. cantrip
3. +1 action
4. trash itself?
5. village (admittedly better than cantrip)
6. Alt-VP: all worth 2 or less
7. nothing
8. terminal draw 2 coppers
9. nothing
Good point worth pointing out. The third piece is to consider the likelihood that the best/average/worst case will happen.
1. Need to load up on cantrips or Villages, ideally trash down a bit. Controllable things, but setting them up takes time and work.
2. Easy to miss, but you can increase your chances with the right deck or enablers.
3. Need a Province (hard to do early) and need to collide it (also hard to do; easier if you invest in enablers).
4. Almost always luck-based and difficult to do. The luck can be mitigated by deck size and enablers.
5. Board-dependent, but you can't activate them early, and raising them to Level 3 might very well mean the game is almost over.
6. Board-dependent. If these aren't good buys, you don't invest in them in the first place, so there's very little downside. But making them work
takes work, and work is worth rewarding even in the absence of significant risk that they'll utterly fail.
7. Deck control can make this work, but you still get orphaned KC's now and then.
8. Trashing enables this. See #6.
9. Luck-based, but the luck can be mitigated by deck size and enablers.
Consider all three things together, and you can figure out the right balance. As I said, these are very difficult cards to make work right. It takes a lot more playtesting than the usual card and probably benchmarking as well.