Do note that the maintenance cost thing will also essentially break retinue balance by halving the amount you can get.
I feel that mercs shouldn't cost extreme amounts to hire, which is why I halved it in the first place.
If practice, I need mercs to either crush the main enemy army, to smash a few lands quickly (if war is large) or to siege a small land you couldn't otherwise (typically count v count). In all these instances, I need mercs for at most 3 years, usually more like 2. So the upfront cost is the main deterrent cost for me.
To cite an example in my single county game, in which I DoW for fabricated claims, I hired the Irish band. In vanhilla it is 95 gold to hire and 6.75 per month. I need one year to win the war (3 months to get mercs to full strength, DoW, smash their army in 2 weeks, then 9 months to siege the land down) So the cost total cost is an initial 95 up front and 81 over 12 months. Seems balance. The catch, though, is I get a good 10 gold from the siege and over those 12 months I'm also earning (vanhilla values again) around 1 gold a month, even with levies. So in practice in terms of "negative balance" which is what really matters, it is 95 up front, and an additional 60 gold. So in this case, it is a 3 / 2 split.
In a larger game, the mercs go from "whole army" to "significant addition". In this case, the mercs are used not because I couldn't win otherwise, it is used so it takes what would be a 10 year war due to sieges down to around 3 due to storming castles. While on paper in this case the total cost of the merc may be more due to upkeep (the balance point is a bit over 14 months). However, this is over 3 years, where passive income makes the actually "dent" smaller, and those sieges rake in income as well. Again, the upfront cost is what really makes the impact, the upkeep is prohibitive only during peace.
As for maintenance costs and it wrecking retinues, I don't like how increasing unit costs affects retinues (not that it is hard to fix, but it is very time consuming), but I don't know how to increase levy_costs as a coefficient since I cannot find such a modifier. Is there such a place where levies costs themselves are increased, as opposed to individual components of it? Of so I'd much rather do that, but I can't find it. If I could I'd increase that and do the merc cost separately. Or I could do a at_war modifier to reduce income rate, but I don't like that so much for it reduces the value of economic buildings while increasing levy costs makes economic buildings valuable.
"Reduce vassal levies cost to 0.1
Reduced liege-vassal maintenance cost to 0.65"
I've got no idea where you're modifying these, though one I assume is LIEGE_LEVY_COST_MULTIPLIER.
Typos, yikes. The first one is vassal mercenaries, in effect making their costs the same while normal mercs increase. The second one is the one you correctly refereed too, which as far as I can tell simply reduces vassal costs when liege uses their units.
