Is CPU mining even worth the Ether?
Some numbers - running ethminer on my CPU gives me a hashrate of 0.248 megahashes per second (MH/s). For comparison, each of my over-clocked AMD R9 390X GPUs gives me a hashrate of about 31.58 MH/s. My GPU gives me 127x the hashrate of my CPU. It is a lowish end CPU AMD FX(tm)-6350 6 core processor and with motherboard + CPU + disk drive + fans consumes about 200 Watts, or 4.8 kilowatt-hours (kWh) per day.
Plugging these numbers into https://etherscan.io/ether-mining-calculator gives me an expected earning of ~ 0.004790 ETH (USD 0.06) per day or 0.033536 ETH (USD 0.39) per week. My cost of electricity @ ~ USD .17 per kWh is USD 0.816 per day which is 13.6x the value of the ETH earned.
Compare the above to my 2 x R9 390X GPUs mining rig. Hashrate is ~ 62.8 MH/s. Again from the mining calculator, my expected earning is 1.210311 ETH (USD 14.16) per day or 8.4722 (USD 99.12) per week. Mining rig consumes ~ 700 watts. This is equal to 700 x 24 = 16,800 watt-hours per day, or 16.8 kWh. 16.8 kWh x USD .17 per kWh = USD 2.85 per day in electricity.
Just remember that the cost of your GPUs may possibly not be recouped by the time that the Ethereum network switches from proof of work to proof of stake when GPU mining will be obsolete.