I tried to do the math on this too. First of all, I'm not sure if the number is 20,000 USD or CAD (Since OpenBSD is based in Canada not the US). Next up is the fact that many of the machines are older non x86 machines that are not power efficient. For example when the SGI/AlphaStations/VAX/SparcStations were produced, focus was on MHz not power utilization. Finally, I think the project might use some type of uninterruptible power supply (UPS) as well as network switches, etc.
So by your math you're looking at CAD 20,000 = EUR 13,500 which at EUR 0.20 per kWh would buy you 67500 kWh = 7.7 kWh.
Now the project has supports about 20 architectures. And there are dedicated machines used to build the base system and dedicated machines used to build ports so at least 2 of each machine. On top of that there's probably an NFS server to host the source code, some UPS, network switches, etc, etc. So say about 50 machines total.
So 7.7kWh / 50 machines gets you to 154 watts per machine. I do believe they are on 24x7 as there are daily builds for many architectures, etc, etc. 150 watts is not unreasonable power consumption in my opinion.