Well you can do the math yourself. Each geothermal is 20EU/t.
Batbox = 32UE/t
CESU = 128EU/t
MFE = 512EU/t
MFSU = 2048EU/t
Remember that you can always set up several of the cheaper ones in parallel instead of making one expensive one. For example you can have 6.4 Geothermals per CESU.
Personally I would make no more than 26 Geothermals. You then produce enough to max out a MFE when you can eventually can afford it. The lava drain from them all running at once will be enourmous, and I hate non-reneawable energysources even if it is just a matter of moving the pump. So instead start saving for solar panels or windmills if you live somewhere high enough for that(and have compact windmills).
