If you need more money set up some Lord Mayors or a Doge if you can. Some Doge's can bring in over 200 in income. Even small Doges easily bring in at least 5-10x the income that Dukes do. You will sacrifice some troops but the extra gold can buy you mercenaries and help to upgrade your holdings which can make up for lost troops.
"Is the a place where it breaks down the expenses? "
Just go into your troop tab. All expenses come from either your troops or ships, your retinue, or mercenaries. You can view how much you spend on all of these in the troop tab. Ships are the most expensive. Your vassals are free but the once from your demense cost a good bit of gold so try not to use them if you are short on gold. Retinues only cost gold when they are reinforcing. Mercenaries are expensive though and early game they can easily make you go bankrupt. Mid to late game, with 1/3 of your Duchy's consisting of Doge's, you can easily hire all the mercenaries if you wanted and have plenty to spare.
The early game is where you really need to watch your gold. Make sure the only holdings you upgrade at first are the castle towns. I get the first two levels of castle towns before I build any other holdings. Adjust your tax policy also. It's easy to keep clergy happy so you can raise their taxes. Mayors are the real money makers though early game. I'd raise their taxes at least one level. Two if you have a good ruler.
Sort your vassals by income starting with the highest and make sure they all have positive opinions of you. If they are negative you receive no gold.