I personally don't think it's enough. Even if you build a mill (which takes a long of denar, and a long time to build) it gives a whole 5% increase... On 500 denar, that would be an extra 25 denar a week... woo! Enough for ONE of my 100+ huscarl.
I control Dhirim and ALL the surrounding castles and fiefs, yet I can't pay my army of 100+ nord huscarl. Of course, there is an error with castles it seems. Even though I show a profit with castles, I lose some 2000+ denars... When I give the castles away to a vassal, I then make money (assuming I have enoughh fiefs minus those castles). This makes no sense and I did report it as a very possible bug.
But, I think that you should be able to increase a fiefs income more than once... Or maybe a mill should increase a fiefs income by 5% per month after it's built.
Anyways, long story short, I agree we need more income.
Mazhron