A network audit of nodes is performed every 30 minutes. Currently, each node that passes the audit is entered into a lottery for the reward (1000 TNT), so 48 rewards a day, and ~1440 rewards / month. Each node's probability of winning is dependent on the number of nodes on the network (3000 nodes atm... so a 1/3000 chance to win every 30 minutes).
1/3000 chance looks very slim to me... it would be nice if there's a fixed reward for hosting masternode...