From DominionStrategy Wiki
Overpaying is a mechanic in the Guilds expansion. Some cards allow the player to spend more coins than their cost when buying them, for additional effects. Such cards have a "+" added to their cost symbol. Cards having an overpay effect are:
- Stonemason, which allows you to gain 2 Actions each costing the amount you overpaid.
- Doctor, which allows you for each you overpaid to look at the top card of your deck; and you can choose to trash it, discard it, or put it back.
- Masterpiece, which allows you to gain a silver for each you overpaid.
- Herald, which allows you for each you overpaid to look through your discard pile and put a card from it on top of your deck.
 Official Rules
- Some cards can be "overpaid" for. The costs for these cards have a "+" next to the coin symbol. A player may pay any additional amount for such a card, and then gets an effect based on how much extra was paid.
- Potions (from Dominion: Alchemy) may be used in overpaid amounts if desired, although this is not always meaningful.
- Debt (from Dominion: Empires) cannot be overpaid.
- Players may choose not to overpay, even if they have extra coins, but cannot choose to overpay by ; to overpay, a player has to actually pay more than the cost.
- The coins used to overpay are gone after spending them to overpay; they cannot be then used to buy something else.
- Overpaying happens when a card is bought, which is before it is gained.
- Players can only overpay for a card when buying it, not when gaining it some other way.
- The "+" is just a reminder; a card with "+" in the cost still has its normal cost for all purposes. For example if a player plays Haggler (from Dominion: Hinterlands), then buys a Masterpiece, overpaying, Haggler will still gain them a card costing less than , the cost of Masterpiece. Similarly, Masterpiece could be the Bane card for Young Witch, since it costs .
- Reducing the costs of cards via cards like Bridge (from Dominion: Intrigue) does not interact with overpay; for example, if you play five Bridges and have total to spend, Herald would cost , but if you bought one the most you could overpay for it would still be .
 Secret History