Ehow.com shows this...
http://www.ehow.com/how_109894_purchase-keg-beer.html
Decide how much beer you need. A pony (half) keg is 7.5 gallons and serves 27 people three 12-ounce servings each, and a fullsize keg is 15.5 gallons and serves 55 people three 12-oz. servings each.
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So according to that we will get 165 12oz cups which works out to about $3.75 a beer not including the bartender prices.
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