I would look at the world or empire, or whatever's GDP. That is something already available, to an extent, in Traveller, particularly the more recent versions.
Once you know what the political entity's GDP is you have somewhere between 1 and 30% of that to spend on a military. Going over 30% pretty much involves eating the economy to supply the army. I'd say 1 to 5% for peaceful, more democratic entities, while 5 to 15% is typical for ones facing a known threat. Above 15% you need either a dictatorship or an actual war to maintain that level of spending.
Since all of the design systems in Traveller include costs, it's easy to see then determine what the equipment will run. Add in the cost of maintenance and ammunition, then determine the cost of the troops, again based on what they're paid per month per the rules.
Another factor to be considered is the government type, with the lower the level of government type, the less likely the government is going to put money into the military. Take a look at US military spending prior to World War One and World War Two, and you will see exactly how little the US was willing to put into a military. The same thing held true in England during the mid to late 1930s. Then there is also the matter of population. A very high GDP with a low population means a limited number of personnel devoted to the military. In a peacetime situation, you will have less than 1% of the population in the military, and often less than that. The US Army in 1939 was 188,565 total personnel, with that figure from the US Army Official History. The US population was over 100 million. The maximum number of military personnel a nation can afford to put into uniform without breaking the economy is 10%, and that does assume a large input of women into the workforce.
One exception to the low government level would be the Civil Service Bureaucracy, which also will work against defense spending. Religious government would be a toss-up, depending on how you view religion.
Then there is the number of support troops required to keep the combat troops going. That number will range somewhere between 2 and 4, unless you are talking aircraft, in which case the number will be somewhere around 10 to 25 per aircraft.
Generally, spending less than 2% means that the military is for show only, with very limited staying power. Five percent is a reasonable peacetime figure for a government facing some form of threat, with 10% meaning that the threat is quite high. The UK in World War 2 managed to get to 60% for one year, but that was eating into the capital base and did severe damage to the economy.
Remember to that the level of defense spending would have to include any form of space force as well, which would tend to be capital expensive, as well as costing more it terms of manpower. Considerably more skill and intelligence is needed to run a fusion or fission plant verses carrying a rifle. Those will cost more to enlist and retain. The US Navy did much better in terms of budget allocation during the years between World War One and World War Two for several reasons. One was that the Navy was viewed as the first line of defense of the US. Second, ships being built in shipyards meant jobs in Congressional Districts, which helped get Congressmen re-elected. The same was somewhat true for the Royal Navy, except they got hit a bit more by the Depression, and actually reduced the sailor's pay, which got a tad sticky. Throw in the RAF, and the Royal Army was on the short end of the budget.
If you include spending for a Space Navy, that force is going to get the major portion of the budget, somewhere around 60 to 70 percent, with the Army getting what is left. If by "Army" is meant the entire surfaces forces of a planet, then you will have budget battles among them as well. The safest assumption is that the ground troops will get the least in any budget.
