I'm running a vanilla minecraft server for 4-6 people on the Raspberry Pi 3 and it runs beautifully, never had any problems. Pi2 was a bit slow at times, but the latest Pi3 has enough horsepower to support that many people without a problem. So really, setting up a minecraft server costs $30 for the Pi + $10 for the sd card. At 5W power usage it can stay on all the time.