If you plan to serve a lot of people, kegs are the way to go. Solo cups are optional but a friend who knows how to tap one is practically essential. Since a beer keg is just the aluminum vessel that carries the beer, you can choose your preferred drink. So, the question everyone asks—how much is a keg—is determined by your beer choice. Just try not to get too excited about the possibilities. Unless you're planning to drain the whole thing yourself (not advised), think of what your guests will enjoy.
Kegs come in a handful of sizes. But the important thing is knowing the amount of beers in a keg:
Half Barrel: Approx. 124 pints (15.5 gallons)
Slim Quarter: Approx. 62 pints (7.75 gallons)
Quarter Barrel (AKA Pony Keg): Approx. 62 pints (7.75 gallons)
Sixth Barrel: Approx. 41 pints (5.16 gallons)
Cornelius: Approx. 40 pints (5 gallons)