In a big-money kind of deck, there's really two concepts you need to be aware of; the first is money density, the second is opportunity cost.
At the end of my previous article, we may be left with an interesting little question. The analysis of buying a second smithy shows that it should be good for our deck pretty early on, right? Like, look at this deck: we've got 7 copper, 1 smithy, 3 estates, and 2 silvers. Our effective money density is around $0.917, we've got around a 40% chance of hitting a 7 card hand... adding a second smithy would decrease our effective money density to around $.846, it's true, but significantly increasing the chances at getting two more cards in our hand is worth it, on the analysis, right? Well, if the choice were between buying the second smithy and buying nothing, you'd be right. But it's not. Any time you can buy a smithy, you can buy a silver instead (okay, the silver pile could be run out - extremely unlikely). And if you buy that smithy, that stops you from buying a silver. The correct play here is for the silver, not so much because of the collision problem (though that makes putting a free smithy in your deck barely worth it in the short term), but more because of opportunity cost, i.e. you have to consider what you buy in terms of what else you could have bought, not in a vacuum.
This is actually an important way of looking at all of dominion, not just Big Money, but I think it's easiest to understand in big money, because of the money density being available. So if you're trying to decide whether or not to buy a market, you can't just look and see whether that's good for your deck, you have to see if it's better for your deck than the alternatives.
One nice little way to look at this is with potion-costing cards. Since whenever you buy a potion, you could've gotten a silver, it's generally true that any time you have $PX (where $P means you have a potion in play and X is some coin amount), you could have bought something costing $X+2. For instance, if you buy a possession, that could have almost always been a province if you'd gotten silver instead. Your alchemists could have been laboratories (hey, that actually makes a lot of sense), your familiars could have been witches, etc. Now, whether or not you should go in for potions for these cards has a lot to do with variance and the usefulness of the potion later in the game, but it's a tremendous illustration of opportunity cost in action. The opportunity cost of buying a potion is a silver (or any other $4 or less cost card), and that silver could have gotten you a province; instead you have possession (unless it's very early in the game or there are engine decks at hand shooting for megaturns, you'd rather have the province).
Perhaps the simplest, first way many players need to realize the importance of opportunity cost is with the 'village idiot'. Villages, like any cantrip, can't possibly hurt your deck, since they replace themselves, right (apart from being able to draw them dead)? This is the thought process a lot of people go through early on. This may be true, but that doesn't mean you should just gobble up all the villages you can; if you start of the game buying village after village after village, you'll have a whole bunch of unused actions lying around, with no action cards to use them on, but more importantly, your deck won't have any buying power, because the opportunity cost of buying a village is a silver.
Another great card to look at through the lens of opportunity cost is Hoard. Hoard could have been a gold. So any time you buy a duchy with hoard, you could have bought a gold instead. Now you are gaining that gold as well, it's true, but you have to look at when in the game you are... do you want a 'free duchy' in your deck yet? Maybe so, maybe not. To say nothing of the times where the opportunity cost of going Hoard over gold knocks you down from $8, buying a province, to $7, settling for a duchy and another gold.
Yes, I'm saying that Hoard is overrated and misused.
The concept of opportunity cost extends beyond just once through the deck as well. That silver that your village cost you is going to hurt your buying power now, and on the next reshuffle, and on the next reshuffle.... Furthermore, the buying power reduction that you feel now is going to make you have to buy something worse now, which is going to further hurt you on the next reshuffle, and then that further reduction on that reshuffle will hurt your buying power even more on the next reshuffle... this compounding effect is why the early turns are generally more important than the later ones.
And you can further extend this paradigm to what I call implied opportunity cost. If I buy a chapel on turn one with $3, there are lots of costs to me. I have the $2 that I spent for it, the silver (or other 2- or 3-cost card) I could have bought instead, the cards that will no longer be in my deck once I use it, plus the turns that I'm using it for, on which I probably won't be able to buy things, plus it being a dead card in every hand after a while. Now, the third one of these, getting rid of the coppers and estates you're trashing is probably actually a boon more than a cost. And very often, it's a really big boon. But it does take the turn you're buying the chapel, plus probably two full turns of chapelling things, plus another turn which is partially hampered by trashing, plus all the times you have a worthless chapel in your deck. Which is all to say, chapel, strong as it is, is not all that great for big money.
Of course, this is just the tip of the iceberg, as to a certain extent, the entire game is opportunity cost upon implied opportunity cost, and how they all compound.