It's fascinating how different our playstyles are, army camp centric vs merc centric perhaps is the best way to sum up the difference, b/c I think we both have high hospital use in common (we're both making use of hospitals and maxing them out on each map). It's fair to say that my play strat is something I developed to minimize the dependency on army camps b/c I got tired of waiting for armies to accumulate, so I found ways of getting armies quicker or reducing the # of armies needed (hospitals). Maybe our strats are equally effective. Not sure.
Regarding terminology of "army production" and counting mercs in them: I was actually trying to adjust to your system which I thought including mercs b/c of the stats you posted (# armies from camps, mercs, drafts, caches; you left out JS, hospitals, so assumed that what you posted was what you calculate against). If that's not how your method works, then my bad & my misunderstanding, and just ignore what I wrote above regarding your calcs.
The calculations I actually work with are "armies required to clear a level" which includes army camps, caches, drafts, mercs, JS, hospitals. It's the sum of all of these which make up the total # of armies required to clear a map, and that's what I measure against. And I find maximizing Hospitals is the biggest benefit, so I prioritize that. Buffing hospitals is essentially a reduction of the # of armies required from all the other contributing factors--and most importantly reduces the amount required from army camps, b/c that is the biggest limiter b/c you have to wait to get the armies. JS is the same but it's a fixed value, so other than actually doing joint strikes to get the value from it, there's nothing more need be said about it. I evaluate decisions in this context, not just maximizing value within "army production" from camps, but on "total armies required to finish the current map" and maximizing what brings the highest gains in that context.
So to actually answer your question...
Now about the SC being too pricey to upgrade:
https://imgur.com/a/DfDNXEQ
Upgrading Gislic for 6.48B to get a boost of 810 armies/sec
OR
Upgrading SCamp for 7.22B to get a boost of 4020 armies/sec
what would you chose?
Tbh I'd likely choose neither. I tend to not upgrade army camps much beyond 1B cost (not a fixed figure, depends on the map). In general, if I have that much money, I spend it on hospitals, mercs and sometimes mines if I need specific ores to work towards profitable crafting (to get money to buy more hospital upgrades & mercs). By the time the army camp prices get >1B, I'm typically far enough into a map that upgrading camps doesn't make sense anymore b/c there's not enough time left on that map to get the value from it.
I align with Olja's comment - I prioritize mercs early and mid to get more territory = more money & more buildings (camps, mercs, hospitals, crafters) & caches, etc.
When choosing how to spend money, I estimate the length of time I have left on the level, then calculate the benefit an army camp upgrade will get me in that time vs spending the same money on a hospital upgrade or buying mercs, etc, and almost every time the result is that the benefit of the army camp isn't enough to justify the cost vs the other options b/c there's not enough time left in the level, especially now on my 2nd playthrough and I haven't taken more then 4 days to finish a level yet.
Example:
- Situation: Playing a map, estimate total 4 days to clear, 2 days left, 500 territories remaining.
- Next Supercamp upgrade is your example above, upgrading to 12.5K from 8.48K for 7.22B, so that's 4.02K increase * 3600*48=695M benefit.
- I would prioritize a hospital upgrade that would give me >695M benefit, so a hospital that gave a 'far territory benefit' of >=1.39M.
- I have 2 options to implement this decision: upgrade a hospital upgrade I already have, or use mercs to conquer a hospital I don't have yet. Especially useful when I've used an FB to reveal where the hospitals are, and I can estimate the benefit of the hospitals will bring based on the ones I already have (they increase in value).
- And almost always, hospitals give the better benefit, especially if you count the 'near territory benefits' which buffs the captures near the hospitals, and the "freebee point" where you can capture all territories under the min benefit of all hospitals at 0 cost of armies, which again gets more money from bonuses.