Warning: this will be a big post. I'm using it to think outside of my head (I'm an extrovert.)
What can Mastermind be? If so, how powerful is it?
Let me start with the extreme cases:
+4 Cards, +1 Buy: basically Council Room without the drawback. So probably about $6 or $7 worth.
+$4, +1 Buy: again, basically Wine Merchant with no drawback. So again in the $6 or $7 likely.
+4 Actions, +1 Buy: Maybe crazy but probably not. Without Actions to go with it, 4 Actions is iffy.
+3 Cards, +$1, +1 Buy: The most similar card is, I think, Tragic Hero. so this is probably actually very close to $5 since it replaces Tragic Hero's payoff with a standard +$1.
+3 Cards, +1 Action, +1 Buy: The most similar card is still Tragic Hero, but this is non-terminal so it's around that $6 or $7 mark if not higher.
+$3, +1 Action, +1 Buy: The most similar card is probably Countraband, more or less. $5? $6?
+$3, +1 Card, +1 Buy:
+3 Actions, +$1, +1 Buy
+3 Actions, +1 Card, +1 Buy
+2 Cards, +$2. +1 Buy
+2 Cards, +2 Actions, +1 Buy: almost a final-form city.
+2 Cards, +1 Action, +$1, +1 Buy: OP Laboratory?
+$2, +2 Actions, +1 Buy: Festival
+$2, +1 Card, +1 Action, +1 Buy: Hey look it's Grand Market.
+2 Actions, +1 Card, +$1, +1 Buy: Really Good Village
I got bored of this before I finished the list but I think we can agree that this card either needs to cost $6 or $7, or have an additional drawback, or lose the +1 Buy.
A fun little drawback if this is to go under Tomb Raider would be "at the end of your turn, if you didn't gain a Treasure this turn, trash this"; or maybe "You can't gain this if you didn't gain a Treasure this turn."
The card also has a baked-in drawback, which is that the first time you play it each turn, it's going to be random. But it's going to be a random really good card, most of the time.
For now I'm going to test it at no +1 Buy, with the "if you didn't gain a Treasure this turn, trash this" and it being underneath Tomb Raider. It may need to cost $4 at that point, in which case then maybe it'll get the +1 Buy back, or lose the additional drawback.