A Rector costs 140M and provides 0.2 M income. So that's 700M per 1M income.
A grid being overdriven to +300 percent will add about 1.0 mex of output when you add one fusion. Assuming a typical mex spot making 2 M, then that's 500M per 1M income. So at +300 percent OD, adding fusions is still cheaper than adding Rectors. Of course, you can do a lot more with Rectors.
At +500% OD you get +0.7 mex per added fusion, which is 714M per 1M income. So at +500% OD you're better off adding Rectors than you are adding fusions, even if the Rectors just sit there and do nothing.
Singus are cheaper per E than fusions, of course. Adding a singularity doesn't become more expensive than adding a mass of idle Rectors until you reach +900% OD. Roughly. The idle Rectors are better because they come on line one at a time rather than having to wait until the whole singu is finished, but I'm not going to do the math to figure out exactly where the break-even point is.
And again, Rectors do lots of other stuff too that overdrive doesn't. The metal income is actually FOR FREE, which means it's ALWAYS better to make Rectors than it is to make power plants for overdrive.