It’s pretty hard to generalize this without qualifiers. Electricity can be the most expensive problem, but it requires carefully planned control of the other factors - e.g large scale with highly automated servers, network, and meat-reducing control planes to become true. Otherwise factors such as people and under-utilization can especially dominate smaller and/or less efficient facilities.
Thinking thru the factors, many seem obvious, but are often forgotten/ignored when comparing rental or IaaS costs.:
Space is a fixed cost driven by market rates and maximum Server Capital Costs i.e. floor space. Failing to fill the room increases your cost/server efficiency. Pretty common to run out of thermal/power before you run out space, as equipment efficiency increases through the lifespan of the facility.
Server and Power costs scale together, carry a minimum cost for keeping machines on, and vary based on utilization. Again if the servers aren’t doing work, your efficiency ratio will drop. Larger space typically have pre negotiated power commitments too - failing to consume that carries fiscal penalties. Servers unit costs are fairly cheap, storage not so much. Full utilization throughout capital/lease lifespan is the goal - anything less increases relative cost/server.
Network costs scale with Server Costs, and vary again by utilization - minimum invest rules apply, all servers need at least one network port, as well as upstream Core/TOR/Miniswitch gear. The network gear lifespan is typically longer than servers, but shorter than facilities. It usually incurs annual support/maintenance charges too. Bandwidth charges are variable as expected.
People costs scale with a step-function and numbers driven by minimum coverage requirements, task complexities, and level of human toil. Performing any task on a device by-hand is expensive in most markets - touches on tickets, change management, task time etc. Fully burdened S+R in Western cultures is typically ~2x the salary - a $80K employee probably costs around $150K by the time all the workplace costs, taxes, and benefits are paid. Network folk are typically premium resources compared to DC Ops. Sustainable 24x7 coverage looks like a staff of 3-4 people.