Why do some nations spend so much more per soldier than others? Several nations spend $400,000 a year per soldier, while many more spend than a tenth of that? What do you get for these widely disparate expenditures?
Most nations just spend enough to have a force of armed men who can keep the peace internally, and at least delay an invasion by a hostile neighbor. In reality, where there are hostile neighbors, everyone tends to spend more on the military, and have more troops ready to fight.
Another reality is that most nations are only concerned with self defense. A few nations have the ability to send troops anywhere in the world. This is very expensive, and one nation has the majority of the "force projection" capability. That's the United States. There are only a few other nations with the naval forces, trained combat troops and air transports to do any force projection (Britain and France are a distant second and third to the United States, and the rest of the top ten in this area are way behind Britain and France.)
The United States accounts for about half the defense spending on the planet. That, plus innovative training methods, and lots of recent combat experience, give the U.S. the most effective armed forces on the planet (man for man and overall). All the training, support and force projection means that 84 percent of U.S. troops are in support, not combat, jobs. Norway, which spends about as much per soldier per year as the United States (close to $400,000) has a force that is 40 percent combat troops. That's because Norway is concentrating on homeland defense.
Many other nations have a lower percentage of support troops simply because they cannot afford to provide much support. This gives you the typical poor nation with a military composed of troops equipped with gear that is often not operational, because maintenance costs money. So does training, which means using a lot of that equipment (and wearing it out, and having to replace or refurbish it). You can also save a lot of money by purchasing second hand weapons and equipment, or simply cheaper models of new stuff. Many nations look at their neighbors, not everyone in the world, to determine how powerful their weapons have to be.
Many nations took advantage of the end of the Cold War (in 1991) to buy fairly modern warplanes and armored vehicles from former Cold War adversaries (the United States and Western European nations, and their former foes in Russia and Eastern Europe). But this high grade gear was still expensive to maintain. So many nations bought MiG-29 fighters and Leopard II tanks, but could not afford to use them much for training. Thus the troops were well equipped, but not as prepared for combat as nations with less modern weapons, who were trained. This was the case in the 1960s and 70s, when Arab armies, often equipped with the latest Russian weapons, were defeated by better trained Israelis, who used a lot of World War II surplus stuff.
Many nations have found that much money can be saved by using more civilians, and commercial firms, to do the maintenance and support function. This means using more experienced personnel for the work, and benefitting from the more efficient methods developed in the commercial sector. Many Middle Eastern nations have been doing this for decades, to maintain large stocks of modern weapons. There are not enough skilled locals available, in or out of uniform. But the United States also employed larger numbers of civilians to provide support functions in Iraq and Afghanistan.
Many nations have also discovered that much money can be saved by having common support functions for the different services (army, navy, air force) performed by shared maintenance organizations. This is often resisted by the services, who all feel that they have specialized needs. But the common support operations can handle these needs, and the common support movement is spreading. Simply because it saves money, which can be spent on new equipment and training.
Data to demonstrate the national differences can be found here.