Expand lets you trash a bad card from your hand, and instead gain a good one. It effectively is Remodel's older brother, as it does the same thing except adding up to $3 of value instead of $2 of value to the card. Does this make it a great card? Yes -- sometimes. But MOST of the time, the answer is no.
When to buy ExpandCase 1: PeddlerPeddler is great with any TFB card, and Expand is probably the best one for it. Why is that? Drawing a Peddler and an Expand gets you a Colony (or in a province game you can still get a province). This is extremely effective, as you can get the peddler for very little but trash it at full value for a huge VP Swing.
Case 2: Grand MarketGrand Market is probably the second best card for Expand. Why? Getting GM's is hard because you need not have copper in play. With Expand this isn't a problem, as you can turn your Silvers into GM's, which is more than a fair trade. Also late in the game, the $6 GM's can be turned into Provinces if need be, which can make a double province turn trivial.
Case 3: CursesExpand is one of the few cards that can do anything with curses. Remodel lets you turn them into estates (most of the time), bleh. Upgrade, transmute, salvager, etc. give you no benefit except getting the curse out of your deck, which is ok but too slow and leaves you with a 3 card hand and your turn accomplishes little. Expand lets you turn curses into $3 cards, which is at a minimum always a silver. Otherwise, there are many more $3 cards you want than $2 cards you want, and depending on your deck adding a Village/FV/GH/WW/etc. can be even more effective
Case 4: (Counter) to DukeThis is certainly not easy to execute, but can give you a chance to stop a Duchy-Duke player dead in their tracks. The idea being is you can buy the duchies (or dukes) at 5, and then expand them into provinces. Doing this even 4 times can make it so that you will be able to beat out the Duke player with your province strategy. This requires a semi-small deck so you can turn them into Provinces ASAP. Although this might not be as good as simply going Duke yourself, if you don't want to go Duke it might give you hope.
Case 5: CursersCursers are great early. Late? Not so much! After the curses are out, cursers fall somewhere between useless and a poor silver. However, with Expand that's ok. Witch done giving out curses? Turn it into a Province! Familiar? How about a Possession!
Case 6: Price ReductionPrice reduction allows you to do crazy things with Expand. The first (and often easiest) way is by use of Highway. How? Cards can't cost less than 0. This means for Expanding purposes, cards become closer to 0, so cards that already are 0 (AKA curse/copper) become closer to the other cards. For example, after just 2 highways, you can Expand a curse into a duchy for a 4 VP swing. Another fun way is by use of quarry. Buy actions (preferably of value 5) cheap, and then Expand them into provinces.
Case 7: Strong (Nonterminal) 5's$5 cards fill an important role in an Expander's world. They fill the crucial step between Estates ($2) and Provinces ($8). On every board, there is an option to simply expand via Duchy, but that is very rarely ideal until very near the end of the game. Instead, it is a lot better to turn your estates into $5 cards that you want lots of. Cards that fall into this category are almost always nonterminal, such as Laboratory, Hunting Party, Bazaar, Market and Minion, but depending on the kingdom, terminals such as Torturer, Rabble and Wharf deserve close consideration. Treasury is a special case, as it can go back on top of your deck, and since you are expanding to gain your province instead of buying it, you can keep topdecking.
While these all make Expand have value, what causes it to lose value? When should you avoid Expand?
When to avoid ExpandCase 1: Lack of +ActionsAlthough Expand is a non-drawing terminal (which is generally the type you like with the absence of villages), Expand is not normally the terminal you want to use, as there is normally a stronger one on the board. It is worth noting that in terminal collisions, you can Expand your other terminal, but you have to ask yourself if it is worth losing your other terminal if it is early.
Case 2: If it will be used as a (weaker) Mine0 + 3 = 3. 3 + 3 = 6. 6 + 3 = 9. The synergy between Expand and Treasures appears obvious. However, there exists a card (Mine) that does this better, as the card you gain goes directly into your hand. And Mine costs $2 less. And it's considered a "weak" $5 card to begin with. So expanding treasures is almost always a bad use of Expand.
Case 3: If it will be used as a RemodelAlthough sometimes you need the remodel effect, Expand is not normally the way you want it. Remodel costs $3 less, and is far from a power 4. Yes, expanding your gold into a province to win the game is a nice feeling, but chances are if you had simply bought a gold instead of the expand, you could have simply bought the province outright. So adding $2 of value is less than ideal.
Case 4: If it will be used to no effectThis may seem obvious, but I have seen many players (even some experienced players) fall into this trap. For example, they might expand a gold into a platinum, and then buy a gold. Basically if they hadn't expanded, their deck would have ended up with a gold and a platinum. And by expanding... they end up with a gold and a platinum. Obviously, the expand is being misused in this situation.
Case 5: Large decks/ slow cycling decksLike Mine and Remodel, Expand gets better the more times through your deck the good cards go. With a large deck, you might only see the card you gained 1 or 2 times, so the better card will have little to no impact on your deck.
General CommentsExpand is a card that looks scarily powerful at first, but really isn't all that good. Most of the time it is used as a Mine or Remodel, in which case you are overpaying for a card that isn't all that good to begin with. Perhaps the situation where Expand "shines" the most is upgrading your victory cards. Turning your estates into duchies gains you VP points without additional clogging of your deck. However, even this is often a poor idea as you are gaining points really slowly. Expand is much better in Prosperity games (surprise surprise) but is often a non factor in games with little prosperity, especially when colony is not out. It is important to remember that every Expand you buy could have been a gold, which gives $3. So you have to ask yourself if it is really worth it.
The general population buys this card 68% of the time, which is way too high. The win rate with is higher without it than with it. Expand is useful in probably 1/3 of the games it is available, not 2/3.
Example Game LogsHere my opponent uses Expand, and quite poorly at that. First off, there is no village so it is drawn often several times. Secondly when it IS used, it has little effect (look at turns 14 (where he could have simply bought the colony), 15 (where he could have simply bought the platinum or if it had been a gold, bought a colony), and 18 (where he could have simply bought the duchy).
Here my opponent makes several mistakes, but his use of expanding coppers to silvers only compounds these problems.
Here my opponent uses Expand very effectively to beat my uncontested Fool's Gold strategy. He expands his estates to Labs/Wharfs, and later on cashes into turn his cards into points.
Here my opponent uses Expand optimally (on a board with curses no less!), but the slowness of Expand shows.
Works with:Peddler
Grand Market
Curses and Cursers
Prosperity in General
Mint
Quarry (and other Price Reduction)
Treasury
Conflicts with:Big Money centered boards
No 5's you want (for your estates to turn into)
Bloated Decks
Treasure trashing (much worse than Mine)
Quick games