EDIT: After trying with 3x recruit and 2x maintain, I tried 2x recruit with 2x maintain and it worked better for the AI.
I recently started playing RS again, but I modified the EDU (for 0-turn) so that all units now cost double to recruit and double to maintain. I did this to address two problems:
-Stack spamming by enemy AI later in the game, forcing you to fight multiple major battles every single turn. Now, the enemy can still raise stacks quickly as in 0-turn (allowing the AI to be more effective), but the number of full stacks possible is significantly reduced.
-Ridiculous amounts of money in the treasury. When playing as Rome, after the first few turns I always ended up having so much money that I couldn't figure out how to spend it all (this is mostly because of the tax bonuses given in home regions and the high income from mines, but also from the fairly low unit costs in general in RS). Unless I simply gave it away or cheated using the console, my treasury would accumulate astronomically. I feel with 2x initial costs and 2x maintenance the economy is much more balanced, and if you want to field lots of legions (especially early in the game) then you'll need to pay attention to your economy.
I've been testing this more expensive EDU file with RS on both BI and ALX, and I have to say I like it much better. It makes the 0-turn recruitment far more tolerable. The AI will still invade with multiple stacks and can still recruit defense forces quickly (leaving the challenge and advantages of 0-turn), but the increased costs seem to prevent the AI from throwing multiple full stacks at you every single turn.
Also, the financial situation for the player now makes the game MUCH more challenging. With double costs the economy seems far more balanced. Militarily, I can't afford many a ridiculous number of legions, so I have to use the ones I have carefully. And regarding construction, I can no longer build whatever I want in every single city, which forces me to make tough strategic choices. Provided I keep military spending under control, I can afford to fund expensive construction projects in some cities but certainly not all at once. This ultimately acts as a real check on my expansion: If I recruit lots of legions to steamroll the AI quickly, then I'm not left with enough money to keep the conquered provinces under control with buildings and garrisons.
Finally, the increased costs seem to slow expansion for both AI and player, which in my mind is a good thing.
The EDU file with 2x costs and 2x maintenance is attached below (as export_descr_unit.txt). (The previous file that was 3x/2x is also available as export_descr_unit_3x2x.txt... but you don't need this if you just want the 2x/2x) Note that this is for RS 1.5b. Also, please note that this file is not officially sanctioned by the RS team in any way... it was strictly my creation for personal use and I'm offering it just so others can try it. To use it, you simply need to copy it over the old file in your "Rome - Total War/data/" folder. (Make a backup copy of the old file: "Rome - Total War/data/export_description_units.txt".)