I would day the effort will produce no tangible results....the CPU draws only what power it needs (unless power saving features disabled) but let's say on a wild chance you could save 40 watts.....
40 watts x 8 hours per day x 30.4 days per month x $0.10 per kwhr x 1/1000 watts per kw = 97 cents per month
Again, the use thing....you are not going to bother the CPU's life at stock speeds and minimal loadings.