What size keg should I get and how much should it cost?

Figure drinking from 8 to 11, 2 drinks per person the first hour, 1 per person for each additional hour, 25 people.... that's about 100 beers. I would get a full size keg to be on the safe side. That's around 170 12 oz servings. The cost will depend on your location and the type of beer you choose. Around here a keg of swill (Bud, Coors, etc) runs about $115. A keg of decent beer runs around $150.