The same? Are you sure?
I give you a example.
I pay montly (for example) 100$ bills.
I mine with GPU. I pay montly 150$ bills (+50$ caused to GPU).
I mine with CPU. I pay montly 110$ bills (+10$ caused to GPU).
My question is such as this.
I pay different bills using GPU or CPU.
Is correct?
You didn't mention what hash rates you get, it all depends how many blocks you mine vs. the electricity you consume. GPUs in general have higher hash rates compared to consumed electricity than CPUs, but that on the other hand depends the model of your GPU. GPU also requires some tuning and right parameters to get best out from it.
Here you can see comparison of CPUs and GPUs and their hash rates (Litecoin is also a scrypt coin so performance comparison apply to WDC)
https://github.com/litecoin-project/litecoin/wiki/Mining-hardware-comparison/9d649243a84b1ec607040b08d7e0ae1c795c0f76#wiki-ATI_AMD_GPUs