I want to field a big army but I don't want to spend a fortune, what would be the best tier to stop upgrading?
What do you consider a fortune? Unit upkeep is so cheap relative to income in this game that it really is not necessary to budget- it is going to cost just as much in recruitment if your units are constantly dying so if you have multiple parties make sure their leaders have the right perks for -recuitment and -upkeep and have the parties set to defensive when they are gathering their party so they tend to recruit in the local area you created the party which should match their culture.
Pretty soon though that won't matter as the AI recruits from wherever it goes.
One change I would like to see is that tier 1 units are just recruits- they adopt the culture of whichever kingdom they are serving as they level up. Anything higher than tier 1 reflects the culture of where they were recruited while bandits should have their own tree or the option to adopt the culture of their party leader.
That said, the Falxman, all of the Battanian noble archers above Highborn Youth long before you get to the Battanian Fian Hero are really cost effective.
Rarely are cavalry cost effective upgrading all the way to the final level which requires war mounts so usually stop at T3 or T4 depending on what the armor/weapon options are.
I find Vlandian Sergeants worth the final upgrade otherwise top infantry cost effectives are probably the Falxman, Sturgian Soldier, Trained Infantryman vs archer heavy factions, Imperial Menavliaton vs all other factions (only more efficient unit is the Battian Fians- 20 often get 100+ kills or 5x number).
Sturgian Soldiers only cost 5 upkeep and their next tier upgrade costs 8 upkeep vs the Varyag Veteran that costs 7 upkeep and only slightly more expensive 2 upgrades path from tier 2 noble so if you want to stop upgrading I'd probably prefer Varyag Veterans as they usually live longer and once you have the money saved, upgrading to Druzhinik is superior than Sturgian Spearman. The following upgrade of Sturgian Spearmen to Heavy Axemen is pretty strong though and way cheaper than the Druzhinik without a war horse requirement so it is all about where you want to go in the long run.
Also- recruiting prisoners if you have a deep pool of your preferred units is by far the cheapest option. I try to carry only 2 types of prisoners at a time but often have 20-30 of each time, it slows down the map movement a bit but allows constant recruitment of the exact units I want without having to spend time recruiting which usually speeds up campaign map travel without having to divert for recruits once you get to the point of being able to start sieges or raids to bait out enemy parties rather than chasing them down.
Also- stay away from mercenaries in taverns- their upkeep and recruitment cost are usually +50% vs comparable non-mercenary units.