I made this chart for myself about 2 weeks ago. It shows the ratio between metal/crystal cost and energy produced for the three types of reactors.
Extracting the most useful data from this, here is the optimal order for reactor building (per planet):
MK II
MK I
MK I
MK III
MK I
MK II
MK I
MK I
MK II
If you need more energy than that, your probably playing wrong, but you can figure it out from the chart.
Minor Disclaimer: This chart was computed with a later 1.202 series prerelease, and may be inaccurate, I am not sure. If it is inaccurate please tell me, and I will redo it completely with the latest prerelease.
EDIT: Screw Excel, uploaded an xml spreadsheet instead.