Grand Markets are almost always preferable to Golds. So long as the average card value in your deck is greater than $1, the self-replacing, non-terminal Grand Market is monetarily superior. The +Buy is always nice, and unlike Golds, Grand Markets are affected by Throne Room and King’s Court.
Unfortunately, it’s difficult to buy Grand Market, since only quite strong decks are able to produce $6 without relying on Copper. There’s several ways to do so before your deck gets to that point.
The first, and most obvious, way is with Actions that give money. Unfortunately, it’s difficult to get enough of them together; it normally takes quite a while to draw 3 Festivals in one hand. Grand Markets themselves are great for grabbing more Grand Markets, though.
A better way is with Secret Chamber or Vault, because it basically lets you use Coppers without having to play them. Of course, Vault is better than Secret Chamber for this purpose: because it gives +2 Cards, it guarantees at least $6. In this game, by going Vault/Chapel instead of Torturer/Chapel, I get my Grand Markets out much sooner than he does.
Quarry also helps; it lets you buy the Grand Market when paired with either two Silvers, a Gold, or another Quarry. (If you use the Black Market trick to play the Quarry during your Action phase, you can use Ironworks to gain it as well.) And the Copper restriction on Grand Markets doesn’t apply when you gain the card with Remodel/Expand/Upgrade.
Of course, if you trash your Coppers, buying Grand Markets isn’t a problem. Nor is it a problem if you have a ton of Silvers in your deck, either from Bureaucrat or Trading Post, but having so many Silvers can be a liability, a sign that you’re not ramping up your deck quickly enough.
In all other respects, Grand Markets have the pretty much the same advantages and disadvantages as Peddlers and Markets (though of course beefed up considerably): they are most valuable in dense decks with high average card value, and are vulnerable to small handsizes.
- Secret Chamber/Vault
- Bureaucrat/other Silver-gaining cards
- Actions that give +$ (especially Conspirator)
- Trashing cards (both to clear Coppers and for a denser deck)
- Coppersmith/Counting House
- Opponents’ Mountebanks
- Attack-heavy opponents in general
- Bank, because it depends on large numbers of Treasures, typically Coppers