From the posts above, it seems as though the following could be the basis for the gold calculation. Either:
a) If you invade someone at level XXX, then you can never be offered less than XXX-100 gold; or
b) If you invade someone in the X00's, then the minimum gold is (X-1)00 (e.g fight someone in the 800's and minimum gold offered is 700).
Then any amount above this base level may be related to the amount of minutes since their last 'tribute tap' & the rate of Kingdom Income.
(...with some adjustment in case someone else has invaded them, although as someone mentioned, there seems to be a window of time in which you can battle again and get offered the exact same quantity of gold even if you've just beaten them)