As an aside, don't forget that your character should have the Property trait as well. For 1,200 square kilometers (the number the Companion quotes as being ordinary for a Baron, Title 4), you'd need it at 7 points. Below that would indicate smaller than average holdings (not unusual). Lacking it entirely could indicate a landless title (also not unusual), or a title whose fief was conquered (e.g. a Tamar Pact noble).
But yes, you would then get about 400,000 C-bills a year "deposited into your bank account", which I always interpreted to be above and beyond what's spent for ordinary maintenance - this is after you've paid all of your employees, maintained the grounds, purchased weapons and ammo for the local guard, etc.
In practice, though, you shouldn't really track that number on a day to day basis. Rather, simply use it to mean that your character can buy whatever he wants within reason. Your character doesn't save and scrimp in order to buy a Blazer; he simply decides he wants one and buys it. A fancy hoverbike? Sure, why not. Don't go crazy, but don't even bother tracking things that cost less than about 1,000 C-bills, and allow more expensive items "within reason". Your baron might not think twice about buying a hoverbike, but it's not like he can buy 10 on the spur of the moment, and military vehicles definitely require some consideration.
EDIT: I just realized that this is covered in the rules starting on page 53 of the Companion ("WEALTH AND EQUIPPED SPECIAL TRAIT CHECKS")
