Its (your fief's population - city population in said fief)*crop production constant,

Example:

Fief A
Scrubland (4.2 crop per population if I remember correctly)
Base Population: 3000
No upgrades
has 1 city with 1000 population

So, Annual Crop production from Fief A is (3000-1000)*4.2=8400.

Now, your population has to eat, so your yearly estimated surplus from each fief is Total food produced - Population of fief*4

So, in Fief A's case it would be 8400-3000*4=-3600 (This means that this fief consumes more food than it generates and should raise a red flag)

Taxes is pretty straight forward. For every single population in a fief. You get 1 bulli per turn. Cities is a bit more complicated and calculated differently from your general tax. It's your city population times the multiplier that your city's wealth provides. So, let's say that Fief A's city is a "Fair" city that has a multiplier of 4.

So, you taxes for Fief A would be.
Part 1:
(3000-1000)*1=2000 (This is your tax income from your general fief population

Part 2: Your city is taxed separately from your general fief population
1000*4=4000

So, Fief A's taxes for 1 turn would be a total of 6000 bullis.