Timerover51
SOC-14 5K
A lot of that cost is how much you pay your troops. The US military pays singularly well for their soldiers. Many nations pay far less. Then there's conscription. If that's being used, the troops may receive very little pay in many cases.
Throw in the cost of quarters, food, etc. Depending on how this is handled the cost could be low to exorbitant. What if the Imperial Army et. al., generally had troops occupying some world build their own quarters out of local materials appropriated for that purpose? That's very common throughout history. Garrison troops might even be required to raise some portion of their food supply in some cases.
So, the cost per soldier could vary considerably.
You need to consider the cost of High Tech Level equipment and troop training, and also the very wide range of environments that the Imperium operates in. One worlds with tainted atmosphere and Vacuum to Very Thin atmospheres, you will need pressurized barracks or barracks with filtered air, with any operation requiring troops training in pressure suits and pressurized vehicles. The US Army in World War 2 determined that it was cheaper to supply pre-fabricated housing overseas than to try and build with local materials, if local materials were even available.
Typically, military personnel are not enthralled with unfamiliar food, which you may have to feed if dependent on local resources. Mutton, widely eaten in Great Britain, Australia, and New Zealand, was not regarded as acceptable by US troops. Canned mutton was even more disliked, as was Australian "Bully Beef". The Japanese supply system assumed that the troops would grow their own food and to an extent live off of the land. That did not work well on the Pacific islands.