The network is currently configured for 1.5 BTS to be created per block. With the current network state of 25 witnesses and 3-second blocks, that gives 1.5 BTS every 75 seconds per witness, or about 51840 BTS of income per month to act as a witness.
Is this the correct approach to calculation? An interesting side-effect is that adding a witness will keep network costs the same, while diluting the individual pay for each witness.