Here is a comparison of an intel cpu with nvidia gpu.
CPU OS Threads Speed (h/s)
i5-2400 CPU @ 3.1 GHz Win10 4 791
Gtx 1080 11gb Ubuntu 400 400
Source:
https://github.com/polyminer1/rhminerRandomHash was design to be much slower on GPU.
There is alot of rooms for optimizations toh, but it will never beat a cpu in term of $/hash.