Cost
Every card in Dominion has a cost indicated in its lower left corner. Usually this is some number of coins, symbolized . The cost of a card is the number of coins necessary to buy it. In order to be able to buy a card, you have to have played Action and Treasure cards or coin tokens generating at least the requisite number of on the current turn (and not spent them on buying other cards).
The costs of some cards from Alchemy include a Potion, symbolized , in addition to—or instead of—some number of . The and components of costs are orthogonal; in order to buy a card with in the cost, you must have played a Potion card in addition to sufficient Actions and Treasures to produce the requisite number of .
Other symbols may appear in costs. An asterisk (e.g., ) indicates a card's cost is not what it seems. Cards belonging to piles not in the Supply have asterisks in their costs to remind players that, despite having a cost, they cannot be bought. Peddler has an asterisk in its cost to remind players it has an effect which changes its cost. A plus sign (e.g., ) indicates that a card may be overpaid for. These symbols are only reminders and have no actual effect on the cost of a card in any of the situations described below.
A card's cost can serve other functions in gameplay, outside of its role in buying the card. For example, most gainers and several trashing attacks can only gain or trash cards of specified costs; most trash-for-benefit cards have effects that depend on the cost of the trashed card; a card's cost determines whether it is eligible to serve as the Bane for Young Witch or be imitated by Band of Misfits. For this reason, all cards have costs—even those that it is never possible to buy, such as Shelters and Prizes.
Some cards have effects that temporarily change the costs of some or all cards—Bridge, Quarry, Peddler, Highway, and Princess. When these effects occur, costs are changed for all purposes—not only the cost of buying cards is changed, but so are other effects that depend on cost such as those listed above.
In general, more "powerful" cards have higher costs, but overall power is not the only consideration that goes into determining a card's cost. (Chapel, for example, is often cited as a card whose power is disproportionate to its low cost of .) Costs between and especially are influenced by how useful a card is as an opening, and how desirable or necessary it is to be able to accumulate multiple copies of a card with extra +Buy. Cards with on-gain or on-buy effects typically cost more than their on-play effect would seem to require.
Most kingdom cards cost between and ; there are only six costing more than and one costing . This means that, in most games of Dominion, there is a gap in the supply at those prices. This gap can influence gameplay in subtle but substantial ways: for instance, it means that Upgrade can be used to trash Copper without gaining anything, but cannot be used to gain Province. In the rare games when - or -cost cards are in the supply, therefore, the utility of a card like Upgrade can change dramatically.