It varies from food vendor to faire owned food booth. Probably nothing below $7, for meals. Some snack type foods should be lower. Most "foodies" don't take plastic payment. You could take enough cash, use an ATM with surcharge, or use The King's Mint (coins bought on credit card and any unused may be "sold back" before end of day\season). The King's Mint coins are in $5 increments and every booth must accept them and give back normal American $ as change.
There is a huge variety of food there; the selection makes this question hard to answer. Snack foods like scotch eggs or ice cream cones are under $4 for a normal serving; more substantial still-not-quite-a-meal options like a turkey drumstick or peasant bread are around $5. Proper meals vary wildly - most are between $7-12 a person depending on portion and complexity. There is also a proper sit-down restaurant on the grounds, the Anchor and Mermaid Tavern, with prices (and accoutrements) closer to a TGI Fridays or similar chain-Americana-bar and restaurant.
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