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First its Gridcoin, not Bitcoin, you can't mine Bitcoin with CPU (well, you can, but you won't see anything).
Second: If it is worth depends mostly on your energy prices, which projects you do and of course when you sell ;)
In most cases, if you buy just for the "mining" you will lose money on current prices. If you run the computer anyway and just take 50 more Watt on that CPU for Gridcoin...
And then there are people who pay electricity with their rent anyway, so whatever they use, they don't pay more.

A few more words to LennStar comments, BTC Mining doesn't help humanity. With BOINC and Gridcoin, I'm contributing to a better world by running helpful projects like World Community Grid while also earning rewards. With Bitcoin, it is just a waste of electricity.