In my current campaign, playing as the Brutii, I was at one stage sitting on a bank of close to 400k. I had paced myself through to around 210bc, my construction tree was always busy and I had regular field armies patrolling all my major borders gaining exp from routing rebel forces and minor border skirmishes. I had a line of supply, from my outlying armies to my major retrain cities and kept the armies fresh with a constant supply of fresh troops. I saved a bucket load as I did not create new units ,(except when tech uprades became available or out of dire necessity)just retrained the old. This way I was constantly using hardened veterans. Very efficient.
Then came the time to move and place my bid for supreme power over Rome.
SPQR, as per usual had some very nice , veteran troops led by some extremely capable generals. Rome was hurled into a prolonged and bloody civil war.
My 400k bank was devoured very quickly. I used my polished diplomats well and where brawn was not the most efficient response, brains came in handy. My bribes were eaten up and within 5 years I had doubled my standing army and had a handfull of new loyal provincial rulers/generals. I expelled the Scipii from the mainland and they now hang by a fingernail onto their few provinces on the African coast.Egypt as an ally really helps, as I can count on them to chip away at the Scipii from the West while I lead lightning raids along the coast. As the ai just tends to play the mass production unit game, I find that slash and burn raids work well to halt the production. When I find them weakened sufficiently I will move in for the kill.
The Julii hold half of Spain but are fighting a defensive war, desperately trying to hold what they own. I am allied with the Barbarians from Gaul who with the help of a steady cash wage from myself, are more than happy to harass the Julii whenever they have the opportunity.
Thrace is mine along with most of Scythia.
Anyway, after that long and drawn out speech, my point is, is that with a little conservative play, tight troop management and polished diplomats, you can build your bank up as high as you like, but when push comes to shove, if you wan't to rule the known world you'll need to part with huge amounts to keep yourself on top of the pile.