It may not be the cheapest (in fact, it is the most expensive), but it is certainly the highest eu output of a MK I-O reactor that I have yet seen that does not require SUC.
Stats:
Total EU: 180,000
EU/Tic 90
Efficiency: 1.8
Excess heat: 0
Resources needed:
6 reactor chambers
9 Integrated Heat Dispensers
23 Coolant Cells
5 Uranium Cells
Here's a Mk I-O for 100 EU/tic, just needs one more Integrated Heat Dispenser and another uranium cell: